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Interrupt 20 ( AKA INT 20, int 20h, INT 20H) is the 33rd entry in the standard PC interrupt table. The '20' is hexadecimal notation (hence the use of the H or h, which may actually be required by some assemblers whereas others assume entries are hexadecimal) for decimal thirty-two (32) but counting starts from zero, as is usually the case in Computer Science.
Int 20 is known as "DOS Terminate" and will end execution of the code at that point and return control to DOS (or the DOS-like environment which called it). It looks at the CS register (a standard 80x86 register which was intended to 'point' to the segment where the code was loaded - Code Segment) and returns nothing by way of register or stack.
Simple Answer: this is an instruction used in very small and typically very old DOS programs to terminate themselves quickly and with a minimum of effort, it probably ought not to be used today, research Int 21 instead (the so-called 'DOS Interrupt').
N.B.: This instruction could also refer to other architectures and thus be very different, e.g. AIX, VMS, etc. My answer is PC & DOS specific as this is the most likely context for this question.
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