A motor unit consists of a single motor neuron and the muscle fibers it innervates. So in one muscle, like your quads, you have many different motor units and the size of these motor units goes from small (small number of fibers per motor nerve) to big (large number of muscle fibers per nerve).
When we talk about motor unit recruitment, we need to think about it in terms of the total number of units being recruited systemically vs locally. What do we mean? Well, when you do a bicep curl with max effort, assuming your bicep is the only muscle working you might be working all of the bicep motor units at max capacity. But in the grand scheme of the body, that’s not really that many motor units. Think of all of the other muscle groups!
So when we talk about movement with large motor unit recruitment we think about movements that involve lots of muscles working hard. So things like squats, sprints and deadlifts require lots of muscles working hard and therefore, lots of motor units. This is why you may not be able to perform big lifts at a high intensity too often, because it taxes the entire body quite a bit!