An entitlement is a record of what a person or account is allowed to use. In a subscription app, it might show whether an account can use a particular feature.
Think of a pass that opens some rooms in a building but not others. The pass does not prove who the person is by itself; the system checks the account and then decides which features are available.
Entitlements help an app apply the right access to each account. The app should keep them up to date when a plan or permission changes.