Chiplets are separate dies that come from different wafers. (It used to be called multi-chip modules (MCM) back in the day but now it's chiplets.) You can see the chiplets pretty clearly in photos of e.g. AMD Ryzen.

The benefit of multiple dies on one package is that on-package wires are denser and shorter which increases performance. Multiple chiplets on an interposer is even better.

https://imapsource.org/article/128222-enabling-heterogenous-...