Turning your work is exactly like it sounds; you flip your work in the middle of a row or round so the other side is facing you. We'd like to think it's the easiest technique a knitter can know (many beginners do it by mistake)!
Turning your work allows you to only knit part of a row, turn around, then work back to the beginning. Doing this in certain sequences can add shaping to your project. It's a bit hard to explain the effect in words, but once you use it you can usually tell how it works.
Note that once you turn, depending on what stitches you're using, your yarn might need to move forward or backward before you continue. Before you knit, make sure your yarn is in the back; before you purl, make sure your yarn is in the front.