Anadromous fish live parts of their life in freshwater and parts of their life in saltwater. They hatch in freshwater, then migrate to the ocean and again return to freshwater when it is time for them to spawn. We assume they migrate like this because they find more food in the ocean than they do in freshwater.
To recap: anadromous fish migrate from freshwater to the ocean, and back to freshwater.
Catadromous fish do the opposite. They migrate from the ocean to freshwater, and back again.
Movie by: Natural Resources Wales (CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0), via Wikimedia Commons.