I recently planted some lettuce leaf basil. Question is, what does pinching back mean?

It's much like trimming, but you'll find that the stalks are thin enough to break by pinching them. So instead of using a pair of scissors or secateurs to trim the heads off your plants, you can just pinch the seed heads and/or flowers off. By doing this, you achieve a thicker growth of leaves. Also, the leaves (which, I assume, are what you want from the plant) will be of a better flavour and quality if you prevent the plant from 'going to seed'.