Okay kiddo, have you ever played in a stream before? You know how the water flows downhill and makes little channels and bumps in the rocks? Well, sometimes there are big rocks in the way that the water can't flow over or around very easily. When that happens, the water keeps pushing against the rock and makes the stream bed deeper right where the rock is. That spot where the water is digging down into the stream bed is called the "head cut". It's like the stream is trying really hard to get past the big rock and is digging a hole to do it.
Now, imagine that happening in lots of places in the stream, with lots of big rocks in the way. The stream starts to look different because it has some spots that are deeper than others. This can make the water flow faster in some parts and slower in others. It can even create little waterfalls or rapids. All of these changes to the stream are what we call "stream geomorphology". So the head cut is just one part of that, where the water is digging down into the stream bed to get past a big rock.