Inline functions are not always important, but it is good to understand them. The basic idea is to save time at a cost in space. Inline functions are a lot like a placeholder. Once you define an inline function, using the 'inline' keyword, whenever you call that function the compiler will replace the function call with the actual code from the function.
A WORD OF WARNING: Inline functions are very good for saving time, but if you use them too often or with large functions you will have a tremendously large program. Sometimes large programs are actually less efficient, and therefore they will run more slowly than before. Inline functions are best for small functions that are called often.
Finally, note that the compiler may choose, in its infinite wisdom, to ignore your attempt to inline a function. So if you do make a mistake and inline a monster fifty-line function that gets called thousands of times, the compiler may ignore you.