What is “System Data” and how do I work on cleaning it up to make more space on my phone?

I have a 64GB phone and about half of that storage is currently full of “system data.” I can’t seem to find a clear way to clean that up - I’ve deleted hundreds and hundreds of photos and videos, and most of the apps on my phone recently, but it doesn’t seem to make a dent on “system data.” Any assistance is greatly appreciated!