Comments are lines in the program that are used to inform yourself or others about the way the program works. They are ignored by the compiler, and not exported to the processor, so they don't take up any space in the microcontroller's flash memory. Comments' only purpose is to help you understand (or remember), or to inform others about how your program works.
The beginning of a block comment or a multi-line comment is marked by the symbol /* and the symbol */ marks its end. This type of comment is called so as this can extend over more than one line; once the compiler reads the /* it ignores whatever follows until it encounters a */.
※ NOTES AND WARNINGS:
When experimenting with code, "commenting out" parts of your program is a convenient way to remove lines that may be buggy. This leaves the lines in the code, but turns them into comments, so the compiler just ignores them. This can be especially useful when trying to locate a problem, or when a program refuses to compile and the compiler error is cryptic or unhelpful.