What makes a good slug
Short, lowercase, hyphen-separated, and describing the page: /image-compressor is easier to read, share and remember than /page?id=123 or /the-best-free-online-image-compressor-tool-2026.
Changing existing URLs
If a page is already published, changing its slug breaks existing links. Set up a 301 redirect from the old address to the new one whenever you change it.