Rub your hands together fast for a few seconds and they warm up. That warmth has to come from somewhere - and it’s coming from the motion of your hands. Friction is the resistance that happens when two surfaces slide against each other, and every time it happens, some of the kinetic energy (motion) gets converted into thermal energy (heat).
This is true everywhere. Sliding down a slide, the seat of your pants warms up against the slide. Car tires get hot from the friction with the road, especially after a long drive. A drill bit working through wood can get hot enough to burn. The act of two surfaces sliding past each other always produces some heat.
It’s also why meteors and rockets get so hot in the atmosphere. A meteor entering Earth’s atmosphere is traveling at tens of thousands of miles per hour. Its huge kinetic energy gets converted into heat through friction with the air - so much that the meteor glows white-hot and often vaporizes long before reaching the ground. A returning spacecraft has to manage that same heat with a heavy shield, or it would burn up like a meteor.
