As you go through the game you will meet other people. Some of these people give you hints on where to catch the best fish, while others will have little jobs for you do. These errands normally require you to catch a specific type of fish and then bring it back to the person.
Some of the little quests are required to be cleared before you can continue to the next map or unlock the next errand in the sequence. If you get stuck on an errand, check the Diary in your Main Menu for a hint on what to do next.
Errands can only be done one at a time. The game will not activate an errand if you are currently working on one. When you talk to the errand giver you'll just get a normal conversation instead of the to-do-list when you are suppose to be working on someone else's job.