A claim is a statement that one resource, such as a person or organization, makes about itself or another resource. For example, the statement can be about a name, group, buying preference, ethnicity, privilege, association or capability. The resource making the claim or claims is the provider.
Claims are packaged into one or more tokens that are then issued by an issuer (provider), commonly known as a Security Token Service (STS). Claims are in reality a set of attributes for an entity.
See standard claims for OpenID Connect for more details.