Keycloak client vs user

According to the Keycloak documentation

1. User - Users are entities that are able to log into your system
2. Client - Clients are entities that can request Keycloak to authenticate a user. Most often, clients are applications and services that want to use Keycloak to secure themselves and provide a single sign-on solution. Clients can also be entities that just want to request identity information or an access token so that they can securely invoke other services on the network that are secured by Keycloak

In short words, not only for keycloak but for OAuth and OpenId Connect too, a client represents a resource which some users can access. The built-in clients for keycloak represent some resources for keycloak itself.