This header declares several useful library functions for performing "istream input and output" from a program.
Most C compilers that target UNIX and Linux do not have this header and do not supply the library functions.
The library supplied with Borland's Turbo C did not use the DOS API but instead accessed video RAM directly for output and used BIOS interrupt calls.
This library also has additional functions inspired from the successful Turbo Pascal one.
Compilers that target non-DOS operating systems, such as Linux or OS/2, provide similar solutions; the unix-related curses library is very common here.