Controlled flight into terrain (CFIT) happens when a pilot accidentally flies an airplane into something hard like a mountain, without realizing it. It's like when you're walking, and you accidentally bump into furniture, but this is a lot more dangerous.
Sometimes the weather can be really bad, making it hard for the pilot to see where they are going, like when it's really foggy outside. And sometimes, the pilot can get confused and forget which way they are supposed to go. This often happens when there are lots of other planes around, and the pilot gets mixed up with all the traffic.
To help prevent this from happening, pilots get lots of training and use special instruments to help them fly the airplane safely. They also have to follow rules about flying over certain areas and staying at certain altitudes to make sure they don't accidentally fly into something hard.
But sometimes, accidents still happen, and when they do, they can be really, really bad. That's why pilots always have to be really careful and pay attention to what they are doing.