Assignment rule are used to assign owner to the records based on the condition(s). You can create assignment rule for Lead and case object.
For Example, Each rule consists of multiple rule entries that specify exactly how the leads or cases are assigned. For example, your standard case assignment rule may have two entries: cases with “Type equals Gold” are assigned to “Gold Service” queue, and cases with “Type equals Silver” are assigned to “Silver Service” queue.
Assignment rule are used to assign owner to the records based on the condition(s).
You can create assignment rule for Lead and case object.
For Example,
Each rule consists of multiple rule entries that specify exactly how the leads or cases are assigned. For example, your standard case assignment rule may have two entries: cases with “Type equals Gold” are assigned to “Gold Service” queue, and cases with “Type equals Silver” are assigned to “Silver Service” queue.
Check this:
https://help.salesforce.com/articleView?id=customize_leadrules.htm&type=0
Best Regards,
Sourav