According to the Catholic Church, what did Jesus mean by saying that the least in the kingdom of heaven in greater than John the Baptist?

I do not believe that there are official statements on the subject of Matthew 11:11, at least not at the level of involving papal infallibility. Nevertheless, there are some Catholic writers and Doctors of the Church who have written on the subject of St. John the Baptist as being the greatest born of women, yet remaining the below the least in the kingdom of heaven.

We see in the Psalms that man was made little less than the angels (Psalm 8:4-6).

St Thomas Aquinas quotes the passage of Matthew 11:11 several times in his Summa Theologica for various questions. We must also keep in mind that at the moment Our Lord spoke these words about St John, mankind had not yet been admitted into the kingdom of heaven because Jesus' sacrifice on the Cross had not yet occurred.

Now St. Thomas tells us in one of his questions (Whether there are several orders in one hierarchy?): "The inferior angel is superior to the highest man of our hierarchy, according to the words, "He that is the lesser in the kingdom of heaven, is greater than he"---namely, John the Baptist, than whom "there hath not risen a greater among them that are born of women" (Mat. 11:11). Hence the lesser angel of the heavenly hierarchy can not only cleanse, but also enlighten and perfect, and in a higher way than can the orders of our hierarchy. Thus the heavenly orders are not distinguished by reason of these, but by reason of other different acts.'

Thus one can conclude that Our Lord spoke about St John in comparison with the angels who art in heaven.

In another question (whether Moses was the greatest of the Prophets), St Thomas stated: "Further, it is written (Matthew 11:11) that "there hath not risen, among them that are born of women, a greater than John the Baptist." Therefore Moses was not greater than all the prophets."

As Catholics we believe that, by a divine prerogative from God, Mary was conceived with no stain of original sin. It is not impossible that St. John, although he was not conceived immaculately may have been purified while still in the womb of St Elizabeth! Although not dogma , the Church has not yet pronounced on this subject.

"There is a solid tradition in the Church that says St. John the Baptist was purified of original sin shortly after he was conceived, while still in the womb of St. Elizabeth. So, this episode of the Gospel referring to the child in the womb hearing Our Lady’s voice, understanding her words and loving her is completely credible." - Professor Plino Correa de Olivra

Here is how Wikipedia puts it:

"Some Catholics have held to a belief that John the Baptist never sinned, though this has never been a point of doctrine and is not binding in belief upon any adherent as is the sinlessness of Mary. In her Treatise of Prayer, Saint Catherine of Siena includes a brief altercation with the Devil regarding her fight due to the Devil attempting to lure her with vanity and flattery. Speaking in the first person, Saint Catherine of Siena responds to the Devil with the following words:

...humiliation of yourself, and you answered the Devil with these words: 'Wretch that I am! John the Baptist never sinned and was sanctified in his mother's womb. And I have committed so many sins..." — Catherine of Siena, , A Treatise of Prayer, 1370.[85][86]" St Catherine of Sienna was declared a Doctor of the Church on October 3, 1970 by Pope Paul VI.

The Catholic Encyclopedia has this to say:

"Now during the sixth month, the Annunciation had taken place, and, as Mary had heard from the angel the fact of her cousin's conceiving, she went "with haste" to congratulate her. "And it came to pass, that when Elizabeth heard the salutation of Mary, the infant" — filled, like the mother, with the Holy Ghost — "leaped for joy in her womb", as if to acknowledge the presence of his Lord. Then was accomplished the prophetic utterance of the angel that the child should "be filled with the Holy Ghost even from his mother's womb". Now, as the presence of any sin whatever is incompatible with the indwelling of the Holy Ghost in the soul, it follows that at this moment John was cleansed from the stain of original sin. When "Elizabeth's full time of being delivered was come. . .she brought forth a son" (1:57); and "on the eighth day they came to circumcise the child, and they called him by his father's name Zachary. And his mother answering, said: Not so, but he shall be called John. And they said to her: There is none of thy kindred that is called by this name. And they made sign to his father, how he would have him called. And demanding a writing table, he wrote, saying: John is his name. And they all wondered" (1:59-63). They were not aware that no better name could be applied (John, Hebrew; Jehohanan, i.e. "Jahweh hath mercy") to him who, as his father prophesied, was to "go before the face of the Lord to prepare his ways; to give knowledge of salvation to his people, unto remission of their sins: through the bowels of the mercy of our God" (1:76-78). Moreover, all these events, to wit, a child born to an aged couple, Zachary's sudden dumbness, his equally sudden recovery of speech, his astounding utterance, might justly strike with wonderment the assembled neighbours; these could hardly help asking: "What an one, think ye, shall this child be?" (1:66)." - New Advent.

Now if St. John was in fact purified while in the womb of his mother he would the greatest of those born of women. Mary's immaculate conception still puts her above St. John in the eyes of the Church!