What is the exact uses of REALM term in security?

In simple terms when you want to access a protected resource you need to identify your self with something like a username and you also need to provide something secret to prove that you are not laying, like a password (credential).

Then this information is checked against a repository and based on the information provided you will be given special privileges (these privileges are defined by the role assigned to you and the user group you are in).

The realm is that repository that does all such controlling stuff.

Take a look at this in-depth explanation:

Security Realm

A security realm is a mechanism used for protecting Web application resources. It gives you the ability to protect a resource with a defined security constraint and then define the user roles that can access the protected resource.

I hope this helped!

A realm is a security policy domain defined for a web or application server. The protected resources on a server can be partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database containing a collection of users and groups.

For a web application, a realm is a complete database of users and groups identified as valid users of a web application or a set of web applications and controlled by the same authentication policy.