Real UID/GID are attributes of the user that are set at login by login shell. They typyically stay the same.
Effective UID/GID are attributes of a process that are used when checking access permissions for operations on files or commands.
Real and effective are typyically the same, but sometimes they change when the executable file have it’s set-user-ID bit on. This happens, for example, when a program needs root or other higher level permissions to perform some operations on behalf of the user such as cron program.