How Rails Concerns Work and How to Use Them

Many Rails applications, including the framework itself, make heavy use of concerns. However, if you are just getting into Rails, concerns can be confusing to understand. Why do we need them, how do they work, and when do we use (or not use) them? When I started learning Rails a few months ago, concerns were one of the topics that took a long time for me to understand.The Rails API documentation doesn't help, either. It starts with the assumption that the reader is already familiar with the problem concerns are trying to solve, and goes on to provide the solution, only adding to the confusion.