Lead assignment rules and default owners
Key Takeaway
A lead assignment rule assigns a lead to the user or queue in the first rule entry that matches it. Web-generated leads that the rules don't assign go to the Default Lead Owner, which you set in Lead Settings.
Lead assignment rules automatically decide which user or queue owns a lead, based on conditions you define. A rule can't always find a match, so Salesforce also needs a fallback owner. For web-generated leads, that fallback is the Default Lead Owner, which you set in Lead Settings.
Where to find assignment rules and who can change them
Case assignment rules do the same job for cases that lead assignment rules do for leads. You find both kinds in Setup: enter "Assignment Rules" in the Quick Find box, then choose "Lead Assignment Rules" or "Case Assignment Rules".
You need the "Customize Application" permission to create or change assignment rules, and the "View Setup and Configuration" permission to view them.