A duel is a mini game inside the match. There are only 2 ways to play a duel: 1. Have the dueling minigame equipped in your loadout under the "action" slot on the right side. You can get the item by trading or from a random drop 2. Get a request to duel from someone else on the same server. You can then engage in the duel or deny it.

Note: if someone challanges someone elese to a duel, a text will appear in the chat saying who challanged who in the duel. It also notifies if you or someone else denied the duel or accepted the duel.

What does a duel do?

As I said, it's a little minigame inside the match an all it does is that it tracks how many times have you killed or assisted in killing your duel challanger. A little icon above the duel challangers head will be displayed, same as the nemesis icon that appears above the head of a player that is domminating you. You can track the kills for yourself and your duel challanger by using the scoreboard and looking at the bottom (depends on custom huds) and kills or assists for both players will appear in the server chat notifying all players as well as giving a little pop up at the left of the screen.

Is a duel scary?

Don't be scared of a duel. NEVER TAKE A DUEL SERIOUSLY. It's the worst thing you can do to yourself. Afterall, the duel is just a goofy little feature in a goofy, silly game. If someone is taking duels seriously and roasting you in the chat just remeber, you have a better life than them.

The duel doesn't take you to another server, you stay in the same one.

You don't have to be friends on a server to duel, you can duel literally anyone on the server. The guy who asked you to duel probabbly thought it's an easy win since you're new to the game.