Say you need to buy some milk. You know that you need to go to the supermarket and look for milk in the dairy department. You aren't going to be looking for milk in the tools department or even other areas of the store (such as the fruits and vegetables departments); you limit yourself to just the dairy department.
A result source acts the same. It allows you to restrict search queries to a specific subset of content from the search index by defining a set of rules that must be met by the content in order to show up as a result.
If the result source sounds familiar to you, it is because you have already encountered it before. Take a look at the following screenshot taken from the default SharePoint 2013 search center: