"30MM" can mean a few different things depending on the context. Here are a few possibilities:

* 30 millimeters: This is a unit of length, equal to 3 centimeters or 0.03 meters.

* 30 millimetres: This is the British English spelling of 30 millimeters.

* 30mm cannon: This is a type of military weapon that fires 30 millimeter shells.

To understand what "30MM" refers to, you need more context. Please provide more information about where you encountered this term.