#include <someFile> is an instruction to the pre-processor to find the file named someFile and copy it completely, exactly, into that spot.
So, #include <iostream> is an instruction to the pre-processor to find the file named iostream and copy it completely, exactly, into that spot. Likewise for the file named string.
In C++, these files contain necessary declarations and other code so that you can make use of functions and classes to do with input/output (in the case of iostream) and string objects (in the case of string).