Zone 2 cardio is performed at an intensity where one can maintain a conversation without getting winded, typically while nasal breathing.

“zone 2 cardio is for, those of you that don't know, a type of cardiovascular exercise that you can do while maintaining a conversation without getting winded but that if you were to push a little bit harder, that you would find it hard to complete your sentences. In general, zone 2 cardio is the sort that you can do while purely nasal breathing, unless you need to talk, of course, it's perfectly fine to talk while doing zone 2 cardio.”