Hume's thesis means that all of our ideas depend ultimately on experience (whether "inward" or "outward").
1. Obviously our ideas of colors and sounds and tastes and the like we have from the actual experiencing of them, from, that is, the impressions.
2. Other ideas, such as that of a gold mountain or of a unicorn, derive from not from some single impression, but are compounded of simple ideas each of which is copied from some impression.
It is important to understanding Hume's arguments here that he seems to take there to be two parts of the thesis. First of all, all simple ideas are copied from corresponding impressions. Second, all other ideas (i.e., all complex ideas) are built from simple ideas by the processes of "compounding, transposing, augmenting, or diminishing" [text] these simple ideas. At Section V "Sceptical Solution of these Doubts," Part II, paragraph 1, Hume offers a slightly different list: "mixing, compounding, separating, and dividing."
3. But Hume's claim also means that our ideas which seem more "abstract" are also grounded in impressions (that is, in experience). Our ideas of the infinite (if we have such an idea), of time, of honesty, of God, of authority, of power, of inequality, and of the number 1000 must all be derived from impressions.