Onionskin is a kind of paper: very thin, very light, a little crinkly, and a little see-through, like the dry skin you peel off an onion. People used it a lot in the past for letters, especially airmail, because a thin sheet weighs almost nothing. Offices also used it for extra copies from a typewriter.

You will not meet it much now. Email took the letter, and ordinary paper got cheap and light enough. When you do see the word, it is often in old books, old offices, or talk about how people used to write.

It is only the paper. Nobody in a kitchen asks for onionskin when they mean the vegetable.