The quality scale
Quality runs 1-100. Above 90 you are spending bytes on detail nobody will see. 80-85 is the sweet spot for photographs — visually indistinguishable at normal viewing sizes. Below 60, flat areas like skies start to show banding and soft blocking.
When to use lossless instead
Logos, icons, screenshots, line art and anything containing text. Lossy compression puts visible fuzz around hard edges, and on these images lossless is often smaller anyway because there is little detail to throw away.
Target-size mode
If an upload form enforces a hard cap, set a target in KB. The encoder binary-searches for the highest quality that fits underneath it, so you get the best-looking file that will actually go through, rather than guessing at quality values.
The method setting
Method controls how hard the encoder works, from 0 (fast) to 6 (slow and smallest). It affects encode time and file size, never quality. The compress tool uses method 6 because the entire point is the smallest file; bulk conversion uses a faster setting.
Compress copies, not masters
WebP is lossy and each pass discards a little more. Keep your originals and compress the copies you publish.