Insufficiently means ‘not enough’, or ‘not to a great enough degree’. If you are insufficiently prepared, you did not prepare enough. If a room is insufficiently lit, there is not enough light. It is a rather formal adverb. Though actually, in speech ‘not enough’ or ‘not well enough’ is nearly always more natural. The adverb is useful in careful writing when you want to say the amount or the degree missed the mark — not that something was absent, but that it fell short. It comments on how far something went, not only on whether it happened.