In Salesforce, users have only one profile, but sometimes you're required to assign more than one profile to users based upon business requirements. A permission set is a group of settings and permissions that gives users access to various apps and functions. The settings and permissions in permission sets are similar to those in profiles, but it extends users' functional access without changing their profiles.

Let's start with an example. Sara Bareilles (Profile - TL) is working as a team lead in Appiuss and her manager Rakesh Gupta (Profile - PM) is going on leave for a week, so Sara is responsible for Rakesh Gupta's work. As both of them have different profiles, Sara Bareilles does not have the access that Rakesh Gupta has; in this case, you can use permission sets.