Well-known IdPs are Microsoft Entra ID, Google Workspace, Okta, Keycloak and Authentik. The IdP holds the accounts or connects to a directory such as Active Directory or LDAP, enforces password and MFA rules, and issues signed assertions or tokens.
From the application's point of view the IdP is the single source of truth for login and often for group membership, so permissions in the application can be driven by groups in the IdP.
Since every application depends on it, the IdP is one of the most critical services you run. Certificate rotation, client secret expiry dates and a break-glass login that does not need the IdP should be documented.