What is Claim in api?

 Claims are the user data and they are issued by a trusted source. If we are working with token-based authentication, a claim may be added within a token by the server that generates the token. A claim can have any kind of data such as "DateOfJoining", "DateOfBirth", "email", etc. Based on a claim that a user has, a system provides the access to the page, which is called Claim based authorization. For example, the system will provide access to the  page, if the user has a "DateOfBirth" claim. In short, claim based authorization checks the value of the claim and allows access to the system resource based on the value of a claim.

https://www.c-sharpcorner.com/article/claim-based-and-policy-based-authorization-with-asp-net-core-2-1/#:~:text=Claims%20are%20the%20user%20data,%2C%20%22email%22%2C%20etc.