Motor is an async Python driver for MongoDB.
You should use Motor when you're trying to interact with a MongoDB database in an asynchronous context. When you're making something that needs to be asynchronous (like a web server, or most commonly from what I've seen here, Discord bots), you also want all the database calls to be done asynchronously. But pymongo is synchronous, i.e it is blocking, and will block the execution of your asynchronous program for the time that it is talking to the database.
Thankfully for us, switching from pymongo to Motor isn't too hard, and won't need you to change much code. This process can be roughly summarized as:
Installing can be done with pip - pip install motor