Meaning and paraphrasing

Hello. Could you please help me understand the highlighted part and the meaning of some of the words?

Let us then suppose the mind to be, as we say, white paper, void of all characters, without any ideas:- How comes it to be furnished? Whence comes it by that vast store which the busy and boundless fancy of man has painted on it with an almost endless variety? Whence has it all the materials of reason and knowledge? To this I answer, in one word, from experience.

What is the meaning of WHENCE?

What is the meaning of the whole highlighted part?( I don't have any idea what the whole highlighted part is talking about, though knowing the meaning of most of the words!)