Okay, so imagine a big rock that has been sitting in the ground for a really long time. Sometimes, the rock gets really hot because there is a lot of pressure and heat around it. When the rock gets really hot, it can start to melt a little bit. This is called anatexis.
Now, when the rock is melting, it's like it's turning into a liquid. But this liquid is made of the same stuff as the rock, so it's not like it's disappearing or anything. It's just changing form.
After that, sometimes the melted rock can move away from where it was before and flow into a new area where it can cool down and turn into a new kind of rock. This process is called igneous rock formation.
So basically, anatexis is when a rock gets really hot and starts to melt a little bit. Then that liquid rock can flow away and turn into a new kind of rock.