Full Idea
We distinguish between 'perception', the internal state of the monad representing external things, and 'apperception', which is consciousness, or the reflective knowledge of this internal state, not given to all souls, nor at all times to a given soul.
Gist of Idea
'Perception' is basic internal representation, and 'apperception' is reflective knowledge of perception
Source
Gottfried Leibniz (Principles of Nature and Grace based on Reason [1714], §4)
Book Reference
Leibniz,Gottfried: 'Philosophical Essays', ed/tr. Arlew,R /Garber,D [Hackett 1989], p.208
A Reaction
The word 'apperception' is standard in Kant. I find it surprising that modern analytic philosophers don't seem to use it when they write about perception. It strikes me as useful, but maybe specialists have a reason for avoiding it.