Groups are used for emailing, but also for permissions to access files, folders, and shared drives.
Groups are named based on Location and position, for example PES Teachers, DO Admin, or PMS Custodians.
There's a list of the locations there on the bottom left.
So for each location, there's a set of regular groups, a set of union/contract groups, and a set of Student services groups for people working in SpEd, RTII, etc.
For each of the groups, there's also an "All" group, which covers everybody in the district, for example, All Teachers, All Admin, All Confidential.
You do NOT need to know all of this, nor do you need to memorize all the groups. Do you see that screenshot up at the top? That's me writing an email to the district office. I type "DIS" and a list pops up of all the groups that match, and I can pick one. Literally just start typing roughly who you want and it will likely pop up immediately.