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To fill out R Barzell's comment more and to venture my own answer in the hope of being corrected. It seems like the most satisfying explanation of this is from this source
the reality we think we perceive does not exist except as as a process of knowing. Phenomena, anything that can be experienced, have no reality in themselves. At the same time, there is no "experiencer" who experiences except as a process of mind.
My further understanding of this though is that the Yogācāra school was a very practically based school and any philosophical discussions would have been secondary to meditation and personal enquiry so this kind of statement would have been provisional on this. This would contrast with the rival Madhyamika school which had a greater emphasis on philosophical discourse. Peter Harvey in an Introduction to Buddhism (pp129 2nd Edition) says of this
Madhyamika has an analytical, dialectical approach [...] emphasizing wisdom; the Yogacara's emphasized samadhi (meditative concentration)