The Garbage Collections tab lists all Garbage Collections that were made during the selected time range, as well as the time spent in Garbage Collection pauses.
Garbage Collection pauses causes your application, or parts of your application, to pause execution. This is done to let the Garbage Collector do it's job. More modern Garbage Collectors are better at keeping the pause times low, but it still happens quite often.
The Garbage Collections tab can be useful to see exactly how much Garbage Collection is being done, and to correlate Garbage Collections with latency spikes. For figuring out how to fix it, looking at allocations is usually more productive - allocations causes Garbage Collections.