What does bridge mode actually do on a router?

You may have noticed a setting for Bridge Mode in the back end settings of your router, but when exactly would you want to ...
Ars Technica

Bridge mode

So I am trying hard to find out how to set my new router as a bridge. Most of the guides don't have subs (i'm deaf), and some are only for specialized modes like WISP or ONT etc. Can anyone help me ...