Group Short Name

The Group Short Name is a four-letter short name that lives over the "feed" of that group

When you create a group, you'll have the option (requirement) to create a group short name. This is a four-letter word that is used on the mobile experience prior to the word "Feed"

You can see from this example, our group short name is CMNT: 

NOTE: The short name does not have to be 4 characters long, it can be shorter, but it cannot be longer :)