Top Roping
Top roping is a type of climbing where a climber has a harness on and they are attached to the rope at the top of the wall. At the top, there may be an autobelay, which is a machine that automatically controls the descent speed of a climber when they fall. However, most of the time, the rope comes back down from the top, so that a second climber may control the speed.
With all this information I'm giving you, I still recommend taking a course at a gym to get professional guidance on this skill, as it's something you have to be taught hands on with.
Questions You May Have Getting Started:
How do I get started?
Like I stated above, I recommend taking a course. The gym you take this at will most likely provide you with all the necessary gear for the session, and they will make sure everyone is safe while they learn.
What gear do I need?
For top rope climbing you will need to have or rent a pair of climbing shoes, a harness, and one person will need a carabiner and a belay device.