It’s a financial instrument that’s designed to replicate the performance of an asset (for example bitcoin). You can have the performance of the asset without having to hold it.
Discussion
Does anyone own any of it then?
Yes and no, the asset manager (for example. Blackrock) will either have the own the asset (buy bitcoin) or if he don’t wanna do that, he can buy a swap (another financial instrument that allows you to swap the performance of a asset A with the performance B) that can give him the performance of bitcoin.