From OpenLieroX Wiki
Combo is the use of more than one weapon in a quick time. It's switching weapons to shoot quickly rather than taking the time to choose weapons slowly and fire. To do this you hold your change weapon button, and press backwards or forwards a few times while pressing fire to shoot them all in what seems to be a combination. The reason this is called combo is because you usually combined the same weapon more than once. Examples of these would be Rifles and Mortars.