Fishing is one of the main ways to get treasure maps for Treasure Hunting, food in the form of fish which can be used in Cooking, as well as a certain type of rare equipment. It can be done afk, but will require a pet nearby to protect you from the occasional monster that can be randomly spawned while doing so.
The first and only requirement for Fishing is to acquire a fishing rod.
Fishing rods of various types exist with different effects. The most basic fishing rod and superior fishing rods can be crafted using the Tinkering skill. Bone fishing rods can be crafted by necromancers via the Bonecrafting skill which allows you to angler up bone fish. Others require Blueprints before they can be made, while adamantium fishing rods are very rare drops from monsters and chests.
Fishing can be done on any water terrain tiles in continent maps, excluding Apocalyptic New Sosaria.
There are also special terrain tiles and locations where the experienced angler with special fishing rods can find unique and interesting fishes.