To wysota:
Open Qt Assistant
what is Qt Assistant?

and why we have to make the function const QString?
It is not "const QString" but "const QString&". This "const" is there to improve performance - you basically tell the compiler that the QString won't be modified inside the function. Normally, the purpose of passing params by reference is to modify them inside the function and not "stress" the stack too much ( like when passing by value ).