On the Render Tab, what does the Compression do?

Should I just keep the compression at 90%?
Will my final quality of my film be better if I decrease the compression to %0? Or does it make no difference?

Some output formats may be able to compress the size of the output. This may be lossless (.png) having no visual effect on the output or lossy (jpeg) giving rise to a lower quality image.

But what about the % on my video? What should I set it to?

Here the compression % is defined as “Quality” so you’d be just fine leaving it at 90; or you can bump it to 100 for greatest quality/no compression. Btw, this has always been slightly confusing to me as well.