A restricted picklist field includes only the values that you’ve defined and users cannot enter any value other than the defined picklist values. This is to ensure that the integrity of the picklist data is maintained and your picklist stays clean. Salesforce has introduced this functionality in Spring’16 and it is in Beta.

If you have a Developer Edition org or sandbox org, no setup is required to use restricted picklists. For all other editions, restricted picklists are available as a beta feature, which means they’re highly functional but have known limitations. Contact Salesforce to enable restricted picklists.

Suppose, you have configured a restricted picklist in Sandbox and when you move it to production, you might get an error “Cannot have restricted picklists in this organization.” To resolve the issue, you can contact support to enable restricted picklists in production org.

Here you can find the release notes from Salesforce.