A tag is an in-game cosmetic prefix that appears next to your name when equipped. A tag that you have on MineSantos (for example) cannot be equipped on Sanktburg or New Mineport, unless it is a Global Tag, which in that case it goes across all GTM's. That means that only some tags are global, while others are for each individual network.
How to get Tags
You can get tags from events that GTM hosts! They are usually on holidays and give unique tags every event. Another way to get tags is from /voteshop if a tag is for sale during the week. You can equip a tag by opening your Phone, then clicking on "My Account" and go to "unlocked Tags". You will see a list of all the tags you own which can be equipped. Simply click on one of them to equip it. Below, you can find a list of all the tags that can be owned on GTM.
Valentine's Day 2018/2019:
Another way to get tags is from the Crowbar Crates at /spawn.
Crowbar-Crates 2019 - 2020:
- Jack 'O
Those are all the tags currently in-game. More are added as more events and tag ideas are created. They end up looking something like this once you equip them.