From what I've seen it is basically a managed and updated feed of indexers that it keeps synced with you *arr apps. The *arr apps are the ones that do the searching based on whatever indexers they have imported from prowlarr. They use different criteria like media quality or profiles I think is what they are called.
Searching prowlarr itself would simply provide you with all results from the different indexers, you can filter results/queries if based on different things/tags but I tend to not use it directly myself.
I've found *arr wiki to be really useful when learning how to set things up correctly.