Supplementary groups are extra groups a user belongs to, as stored in /etc/passwd. Users also have one and only one primary group.

What are supplementary group IDs in Unix?

Supplementary groups

A user may be listed as member of additional groups in the relevant entries in the group database, which can be viewed with getent group (usually stored in /etc/group or LDAP); the IDs of these groups are referred to as supplementary group IDs.

What are groups in Linux?

In linux, there can be multiple users(those who use/operate the system), and groups are nothing but the collection of users. Groups make it easy to manage users with the same security and access privileges. A user can be part of different groups.