Let me make a guess, by analogy to what Activity Monitor shows on a Mac.
Your tag line says that you have an iPhone 12 Pro Max. According to MacTracker, that phone has 6 GB of RAM.
Whatever app you are running is telling you that you have
- 5.56 GB Total
- 1.94 GB Active
- 1.92 GB Inactive
- 1.11 GB Wired
- 531.2 MB Compressed
- 58 MB Free
It looks like Total is just the total of all of the other categories. That leaves 0.44 GB of RAM unaccounted-for, but maybe some of that RAM is devoted to iOS or hardware use that this app simply doesn't know how to measure.
I would guess that the Inactive number refers to what the Mac calls Cached Files. That is, the iPhone might have 1.92 GB of RAM that it could reallocate to apps at the drop of a hat, but that it is putting to work caching stuff, for now, just in case the cached data is needed before the RAM is.
If that is correct, the iPhone could presumably dump all of the cached stuff,. Then you'd have 0 GB Inactive, and about 1.98 GB free. You might also notice the iPhone running slower, or the battery charge running out sooner – because by keeping the RAM cache empty, you'd be forcing the processor go back to the slower flash chip(s) more often.