The --watch is on by default.
The site will rebuild. You can look at your log for when this is done. And any buios errors (often why I don’t see changes in the browser because I broke the site)
You can also add this.
jekyll serve --livereload
That will refresh the browser after Jekyll has rebuilt.
If you have issues, restart your machine.
If on windows or using subsystem, you have to add an extra Jekyll flag or gem to support watching. You can try this flag regardless of your OS to see if it helps.
--force-polling