Client Roles  are specific to individual client applications within the realm and each client can have its own set of roles that define access permissions specific to that application

@PreFilter annotation is used to specify an expression that must be evaluated to true in order for a method or class to be included in a security filter chain

 Keycloak user represents an individual entity that can authenticate and interact with the Keycloak server.

 As a result of  Authentication a user gets authenticated and security token granted to them.

Realm in Keycloack is a management entity that controls a set of users, their credentials, roles, and groups

Underlying source code of a open source  can be used, modified and redistributed by anyone and everyone 

@PostAuthorize: It is used to secure a method after it is executed.

As a result of authorization users get access to specific features based on the rules.

Keycloak is based on standard protocols and provides support for OpenID Connect, OAuth 2.0, and SAML

An open-source project is any software project where the source code is made freely available to the public