The letters STL stand for the Standard Template Library. This is a rich collection of standard data structures that have recently been added to the definition of the C++ language.
The idea behind making a data structure library is that we want to elevate the creation of data structures to, for example, that of the I/O library. That is, it is something that is taken for granted, not something you have to write for each new project.
The design of the STL is very interesting in part because it is in many ways anti-object-oriented, and yet it is very powerful, and works well.