Nearly all the greens I grow are considered cut-and-come-again greens. But what does that mean? And why haven't I ever mentioned it before? First think about how salad greens grow: new leaves come from the growing point, which is at the bottom of the plant. If you don't cut off the growing point when harvesting, new leaves will emerge and you can harvest a second time!
Brassicas, like the kale and mizuna above, make really good cut-and-come-again greens. They are fast growing and often have jagged leaves that hide any past scissor marks. Leaf lettuce is not quite as good as a cut and come again green because it is slower growing and because it doesn't hide past harvest cuts well. In the right hand picture below you can see a very flat-topped lettuce leaf that wouldn't look so nice in a salad.
Spinach is a good cut-and-come-again crop because it's leaves are on longish stems. Spinach is slow growing, so plants grown on benches, in flats may run out of nutrients before they are big enough to be harvested a second time.