Sync is just having your devices communicate with each other. So when you save a bookmark, it will also be available on the synced profile on any device. It also would share passwords, so when you add or change a password within the browser on one it duplicates on the other.
If you want to read up on it more, you can check out official articles at links below:
- https://support.brave.com/hc/en-us/articles/360047642371-Sync-FAQ
- https://support.brave.com/hc/en-us/articles/360059793111-Understanding-Brave-Sync
- https://support.brave.com/hc/en-us/articles/360021218111-How-do-I-set-up-Sync
Then I also some basic articles I had written for issues people were experiencing, which may be good to read through as well