What does "STL allocate memory internally" means?

Question

I was reading this answer and maybe because I have never encountered this words, I don't understand what the user was mentioning in the first point of that answer, can someone use simpler words or an example to show what that statement means ?

Answer 1

When you use something like vectors or map ,... it belongs to STL (STANDARD TEMPLATE LIBRARY). you don't need to allocate memory as you do in arrays. In realtime the arrays are not sufficient and we cannot determine size.

STL containers will allocate memory internally, as you add elements to it. so there is good memory management. [if users manually allot, it might be not enough if alloted less or gets wasted if alloted too much memory].