clrscr() is not part of standard C or C++. It was part of the original Borland C compiler as long as the header conio.h was included in the program.

For example:
#include <conio.h> // Include the conio.h file in the program.

The conio.h header is not part of the ANSI or ISO C or C++ language definitions.That's not a problem unless you plan to write portable code that must compile accross multiple compliers and platforms.

If you are writing a console-based program, then clrscr() will clear the screen (or window) that has the console in it. The function works if you are running in a "DOS" type window or if you are running in a true windowing environment that supports a console style window.