It stands for standard template library
It is a set of functions and class that are there to save you a lot of work.
They are designed to use templates, which is where you define a function, but with out defining what data type it will work on.
for example, vector more or less lets you have dynamic arrays. when you create an instance of it, you say what type you want it to work for. This can even be your own data type (class).
Its a hard thing to think about, but it is hugely powerful and can save you loads of time.
Get reading up on it now! You want regret it.