Reverse thrust is what happens when an airplane lands and needs to slow down quickly. Just like when you ride your bike and use your brakes to slow down, an airplane also needs a way to slow down when it lands on the runway.
When an airplane lands, the pilot presses a button or moves a lever to make something called "reverse thrusters" come out of the engines. These thrusters are like giant fans that blow air in the opposite direction of the plane's movement.
Imagine you're blowing a balloon up and then suddenly let the air out - that's kind of what reverse thrust looks like. The fans blow air out of the back of the airplane, which pushes the plane forward and helps it slow down. The pilot can control how much air is blown out so the plane never stops too suddenly.
Reverse thrust is very important because it helps the plane slow down quickly and safely. An airplane going too fast on the runway could be dangerous if it can't stop in time, so reverse thrust is used to help make sure that doesn't happen.