An identity provider (abbreviated IdP) is a system entity that creates, maintains, and manages identity information for principals while providing authentication services to relying party applications within a federation or distributed network - wikipedia 
An identity provider offers user authentication as a service. Relying party applications, such as web applications, outsource the user authentication step to a trusted identity provider. Such a relying party application is said to be federated, that is, it consumes federated identity.
An identity provider is “a trusted provider that lets you use single sign-on (SSO) to access other websites.” SSO enhances usability by reducing password fatigue. It also provides better security by decreasing the potential attack surface.
# Persona IDP
A Persona Identity Provider (IdP) is a domain which directly signs and certifies the identities of its users. Because Persona identities are based on email addresses, any domain which offers email to its users is a natural fit for becoming an IdP - developer.mozilla.org
If you have a domain name, you can become a Persona IdP by implementing support for the underlying BrowserID protocol.