What is System.runAs() Method in Salesforce?

Generally, all Apex code runs in system mode, where the permissions and record sharing of the current user are not taken into account. The system method runAs enables you to write test methods that change the user context to an existing user or a new user so that the user’s record sharing is enforced. The runAs method doesn’t enforce user permissions or field-level permissions, only record sharing.

You can use runAs only in test methods. The original system context is started again after all runAs test methods complete.

The runAs method ignores user license limits. You can create new users with runAs even if your organization has no additional user licenses.

For more information visit :

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_testing_tools_runas.htm