Fishing is an activity in game that is the primary source of fish as well as other items.
While not required, the most efficient way to fish is by using a fishing rod as it uses no energy and has the highest chance of success (although you can fail with it). The second option (using a tree-branch) has a decent chance of success, but uses 15 water and 30 rest. Using your bare hands (10 rest) has a very poor chance of success, and can lead to the Bleeding status, or may spawn an Infected crawler. Keep in mind you do not get a second chance at a specific spot. Like items, all spots refresh one hour after the first thing you collected.