What is "Documents and data"?

I have an iPhone 8 with 64GB of space. It has 2 GBs used for Photos, 18.15 GBs used for Apps and then there's the Documents and Data with is taking up another 21.51 GBs. I've gone to my iPhone under Settings-General-iPhone Storage and deleted some apps I never use and then went into Review Large Documents and deleted 2GBs from it (which BTW, only lowered the Documents and Data file size by .5GBs - half a gig). So how can I clean up the space used by the Docs and Data?

Thanks