what does the 21h function do?

The 21h set of functions is called the DOS interrupt on Intel machines.
It is actually a collection of a lot of functions and sub functions.
For example function 01/21h reads a character from standard input.
Function 09/21h prints a string to standard output. Other functions
in the 21h collection read/write/open/close files and other basics that
DOS provides for you.

: what does the 21h function do?

: what does the 21h function do?

you find some descriptions here:

http://www.geocities.com/emu8086/Help/supported_interrupts.html

only most common are described there.