What $rid is.

$rid is role id. Each field's permission is stored per role.

Where can you get this kind of info? I'm searching weeks for info what tid, rid, mlid etc is.

In Drupal, fields named xid are usually primary keys for database tables. The "x" represents the first letter of the entity whose primary key is xid. For example: nid => node, tid => term, uid => user. Unfortunately, when it comes to third-party modules, you will find several tables having the same initial, and hence will have id fields name similarly.

Do when you see a field named xid, try to find the best fitting database table whose name starts with x.