What does,'I establish mindfulness in front of me' mean?

Thanissaro writes this in his footnotes to https://www.accesstoinsight.org/tipitaka/mn/mn.118.than.html#fn-1 ;

To the fore (parimukham): The Abhidhamma takes an etymological approach to this term, defining it as around (pari-) the mouth (mukham). In the Vinaya, however, it is used in a context (Cv.V.27.4) where it undoubtedly means the front of the chest. There is also the possibility that the term could be used idiomatically as "to the front," which is how I have translated it here.

AN5.75:15.1: Gone to a wilderness, or to the root of a tree, or to an empty hut, he sits down cross-legged, with his body straight, and establishes his mindfulness right there.

Right there as if your life depended on it. Because it does.

I think it might be easier to demonstrate what this passage seeks to combat instead of diving right into how to practice it. Take a couple of minutes and just let your mind wander. Try to run through the last TV show you watched or replay a recent conversation in your head. Let that play out for a minute or two. Once a minute has passed, direct your attention to the tip of your nose. Notice the shift in perception? Where before your mind was diffused and tangled up in itself, now it is directed, collected, and composed.

When our minds are untrained, we spend 99.999% of our lives with our minds floating like dust motes in the air. They flit here and there, directionless, without ever alighting on anything. Buddhist practice tries to combat that and establish new patterns of attention that make our mind more unified and concentrated.

The advice given in the suttas to establish mindfulness in front of you is good advice. If you did the exercise I recommended, you'll see how quickly that shift in attention can occur just by simply (and literally!) shifting your attention in front of you. But don't mistake the practice for the goal. Ultimately what you are after is mind that is collected. In Zen, we'd call that kanshin - the mind with no remainder. This sort of mentation is characterized by it's unity. The normal mind is like a bag of spilled marbles. Kanshin is all the marbles put back in the bag. In kanshin, there is no remainder of marbles outside of that bagged collection. They are all gathered up. Of course, just as there are a number of ways of collecting those marbles, there are several ways of collecting the mind. We can "set our minds up in front of us", but we can also work with a koan, a mantra, awareness of our bodies, a kasina, or our breath. Even the discomfort of hunger or the vigilance of virtue will "keep our minds in front of us", so to speak. So as long as the practice we pick up unifies our minds and keeps us from scattering our mental energy, it will work.

If you want to work with this as a koan, see the following:

Case 16 of the Mumonkan
When the Bell Sounds

Unmon said, "The world is vast and wide.

Why do you put on your seven-piece robe at the sound of the bell?"

Mumon's Comment

In studying Zen, you should not be swayed by sounds and forms. Even though you attain insight when hearing a voice or seeing a form, this is simply the ordinary way of things.Don't you know that the real Zen student commands sounds, controls forms, is clear-sighted at every event and free on every occasion?

Granted you are free, just tell me: Does the sound come to the ear or does the ear go to the sound? If both sound and silence die away, at such a juncture how could you talk of Zen? While listening with you ear, you cannot tell. When hearing with your eye, you are truly intimate.

Mumon's Verse

With realization, things make one family;
Without realization, things are separated in a thousand ways.
Without realization, things make one family;
With realization, things are separated in a thousand ways.

Don't worry about sound just yet. Be the monk putting on his seven piece robe. Where is his mind when he does that? When you truly become that monk, you will see your mind set up in front of you.