Steady aerobic work at an easy, conversational intensity, roughly 60 to 70 percent of maximum heart rate, held for 30 to 90 minutes.
The defining test is the talk test: the client can hold a full conversation but would rather not sing. On a heart rate monitor that is usually 60 to 70 percent of max, and on RPE it is about 3 to 4 out of 10.
The point is aerobic base. Zone 2 builds mitochondrial density and capillary supply, improves fat oxidation, and does it with little fatigue cost, which is what makes it compatible with lifting in a way that hard intervals are not.
Dose it in minutes, not sessions: 120 to 180 minutes a week, split over two or three sessions, is a reasonable target for a general client. The most common error is going too hard, which turns it into moderate work that costs recovery and builds less base.