First-hand describes knowledge that came to you directly — you saw it, you did it, you were there, or the person it happened to told you themselves. Nobody passed it along in between. 'She has first-hand experience of the job' means she actually did the job. 'I heard it first-hand from the doctor' means the doctor told you, not a friend of a friend. The picture behind the word is of something passing from hand to hand: your hand is the first one it lands in, so it has not been changed on the way. That is the whole point of the word — it is about how trustworthy the information is. Second-hand knowledge is 'someone told me'; first-hand is 'I know'. It works two ways in a sentence. Before a noun it is an adjective: first-hand experience, a first-hand account, first-hand knowledge. After a verb it is an adverb: 'I saw it first-hand', 'I learned that first-hand.' Both are common. Now the honest part: 'first-hand experience' is one of those phrases people say when the experience was actually a bit thin. A politician says he has first-hand experience of poverty because he visited a poor area for one afternoon. The word carries weight, so people borrow that weight. Careful listeners notice. Also, do not mix this up with second-hand meaning used or not new — a second-hand bicycle. That is the same word doing a different job, and there is no matching 'first-hand bicycle'; for that we say 'new' or 'brand new'.