I won't go into every single thing that's changed, but I will highlight atleast a few to ease your migration.
One of the most important things, is that the grid system is not changed. Actually a new sized class "xxl" was added, so you can also easily manage how the site will look on huge screens.
Additionally jQuery is dropped, so your site will run slightly faster, since there's no need to load so many libraries/frameworks.
The v5 update brings a bunch of new stuff (read "toys") to play with:
Take note that Bootstrap 5 doesn't support older browsers. Upgrading to the latest browser versions is important to keep your browser and data secure.
To only view the updated stuff, that will affect the projects that use the previous version of Bootstrap (v4) - it's labeled with a red "Breaking" badge.
You will see the word "migrate" a lot in the programming world, be sure that it doesn't only mean "Updating Bootstrap" or "Updating a front end framework". It can also mean "To send data from one database to another one in bulk".
Migrating in the context of this post means just removing Bootstrap 4 (the CSS and JS) and all of it's dependencies: jQuery, PopperJS, from the site entirely.
Then it's just a matter of "Getting Started" with Bootstrap again (make sure the documentation is for the version "Bootstrap 5.0" or above in the upper right).