BIOS/DOS services are called via "software interrupts," i.e. the INT
instruction.  For example, service number 16H, subservice 04H, controls
the keyboard click.  To turn the click off, we would have the code
 
   MOV AH,4     ;  subservice 4
   MOV AL,0     ;  turn off the click
   INT 16H      ;  call service 16

It would be inconvenient to have to add assembly-language subroutines
to a C program whenever we needed an OS service.  So, C compilers for
DOS feature a library call int86().  Here is how we could do the above
in C:

   #include <dos.h>

   union REGS InRegs,OutRegs;

   ...  (intervening code)

   InRegs.x.ax = 0x40;
   int86(0x16,&InRegs,&OutRegs);

(Here is a test of your knowledge of C:  Why SPECIFICALLY do we need
to include dos.h?)

The REGS union form is required for the second and third parameters
of int86().  It has fields for the various 80x86 registers (except
for segment registers, for which there is an SREGS union).  The
second parameter is used to put values in the registers before the
INT is done.  The third parameter is used to retrieve values from 
registers after the INT is done (since many OS services have return 
values, which are returned via registers, though not in the keyclick
case).