For the most part, all the programs you’ve written have relied on combinations of built-in objects and objects that you created yourself. A large part of programming is learning to use code that others have written to your advantage. You can bring their code into your own and then use functions and classes that they’ve already written. You’ve done this a little already in some of the capstone projects.
Why would you want to do this? Often, different programmers need to do the same set of tasks. Instead of coming up with their own solutions independently, they can use libraries that contain code to help them achieve their goals. Many languages allow programmers to create libraries. The library may be included in the language, or it might be found on the internet and distributed separately. Libraries are usually bundled to contain functions and classes that are in the same vein.