Larry454 gives the most correct answer, but I want to add. Older jet engine technology used the 'clamshell doors' to redirect the engine exhaust thrust towards the front of the aircraft, to 'push' against the direction the aircraft is moving to slow it down. Later, engines with higher bypass air ratios, stopped the practice of using clamshell doors and don't bother with the core engine (hot stream) exhaust at all. What the later engines do is slide back some sleeves (trans-cowls) to expose the reverser louvers, which are angled vanes, then put 'blocker doors' up in the engine bypass duct, to prevent the bypass (fan) air from continuing out the back of the engine, and redirect it at a forward angle out the reverser louvers, against the direction of the aircraft's travel.
36 years in industry.