Fishing is the activity of trying to catch fish(Mackarel/Mackerel in English).
Fish are normally caught in the fishing spot, you will notice it once you have Fishing Rod / Hook.
Fishing may include catching stuff other than fish, like wood, metal, and even treasure chest that may contain stuff like Potions, etc.
Why only Mackarel fish in the game? (You may ask)
I'm too lazy to make many kind of fish plus it would troublesome to search or create their icon too as well.