Secure your data and boost productivity by permitting your users to see only the records necessary for their job function. Create restriction rules to control which subset of records you allow specified groups of users to see. Restriction rules are available for custom objects, contracts, tasks, events, time sheets, and time sheet entries. This feature, now generally available, includes some changes since the last release. You can now create and manage restriction rules in Setup as well as with Tooling and Metadata APIs.
Where: This change applies to Lightning Experience in Enterprise, Performance, Unlimited, and Developer editions.
How: To create a restriction rule, navigate to Object Manager in Setup. Select the object that you want to add a restriction rule for. Click Restriction Rules. Name and describe the rule and activate it. Select a user field and choose filter settings to determine which users the rule applies to. Then, select a record field and choose filter settings to determine which records are accessible.
Control Access to Sensitive Data with Restriction Rules (Generally Available) (salesforce.com)