Restriction rules versus sharing rules
Key Takeaway
Sharing rules add access on top of the org-wide defaults. Restriction rules filter the records a user can already reach.
Sharing rules and restriction rules both control which records a user can see, but they work in opposite directions. Sharing rules open up access beyond the org-wide defaults for chosen groups of users. Restriction rules sit on top of all sharing and filter the records a user can already reach down to only the ones that match criteria you set.
How record access builds up
Record access starts with org-wide defaults, which set the baseline access users have to records they don't own. Role hierarchies, sharing rules, and manual sharing then add access on top of that baseline.