I’m going to actually assume you’re an out of context beginner. The earlier responses are right but they aren’t actually helping you understand the concept.

The perfect nozzle has the ambient pressure exactly match the exhaust pressure. This because any excess pressure in the exhaust gases is wasted energy that could have been done. And if the pressure drops below ambient, that means the atmosphere will be pushing in on your exhaust gases, reducing velocity and thus performance.

But ambient pressure changes with altitude. So the perfect exit size changes as you ascend with it. In theory, an infinitely adaptable nozzle would be ideal, but to accomplish that objective requires weight. And the rocket equation tells us that the pounds of mass we have to take to orbit isn’t worth what you would have to spend to get it there .