All of the code we have explored so far is very linear down the file. Each line is processed one at a time and then the program moves onto the next. This is one of the great things about programming: everything the program does can be predicted by stepping through the program yourself mentally, one line at a time.
However, as your program gets larger, you will notice that there are places that reuse very similar or identical code that you cannot reuse by using loops. Moreover, the more code you write, the harder it becomes to know exactly what it is doing. Code comments can help with that but there is an even better solution to both of these problems and they're called functions. A function is essentially a named collection of code that can be executed and reused by using that name.
There are various different types of functions but each builds on the previous type.