briefly, perhaps not 100% correctly - use Google for more details, claims are characteristics provided by OIDC (OpenID Connect) server that describe authenticated user. e.g. Caché by default supports these claims (and more):
preferred_username, email, email_verified, name, phone_number, phone_number_verified, iss, sub, aud, exp, auth_time, ...
claims can be used by the access_token to provide additional information about the user passed to the resource server when calling REST method. Some claims are mandatory, some are optional. You can provide optional claims by the SetClaimValue() method.