Mitwits are green robe villagers that have no trades. But they were useless until now! In this creation mitwits can give you tasks. To recieve a task, come close to a mitwit and wait for his monolog to start. After he finishes talking, he gives you a task. If you didn't give him a requested item (a message about what item you need to give appears in the chat), he will not give you more quests. You can see his quest above his head.
Here is the list of all possible tasks
Baked potato (baked potato)
Chorus fruit (chorus fruit)
Enchanted book (any enchanted book)
A pickaxe (any pickaxe)
Nether item (netherrack or quartz)
Prismarine (prismarine block)
A gold nugget (gold nugget)
Nether wart (nether wart)
Here are all possible quest villager variations
If mitwit gets a wanted item, you will complete the task and recieve one of the rewards:
A golden apple
5 levels of experience
2 emeralds
After that you will be able to talk to that mitwit again and get a different task (he can also give you the same task because of randomness).