When a statement in a line of code tells the program to perform the instructions defined in a particular function, this is referred to as "invoking" or "calling" the function. A standard C program always includes a special function called "main()", and program execution starts with the first line of that function. As you will see below, functions often include calls to other functions.
The syntax of a C function is essentially the same as is used in Java, Processing, JavaScript and a host of other programming languages, so once you learn it, you can read and write functions in a bunch of other procedual programming environments.