Rerolling is the action of restarting your account, and using the free gems given to new players to get the thing that they want from the shop. Since they are usually super-rares, or ultra-rares, they have a small chance to obtain it, hence the repetition of uninstalling and reinstalling to delete the account and make a new one to keep doing it. Essentially similar to "rolling" a dice until you get lucky.
Rerolling in Duel Links
Rerolling in Duel Links is to re-open a pack that is given to you as the first and a reward for finishing a tutorial at the beginning of the game. You get three cards, and if those cards are not cards that you want, conduct rerolling.
Rerolling is just for players who just have started to play the game. They perform rerolling to re-open a first free-of-charge booster pack. Do not uninstall the app you have already made some progress in the game!
How to Reroll
In a General Way
Install the app.
A tutorial starts.
Finish the tutorial.
Receive mission reward and visit the Shop.
Open a pack that is given to you for free of charge.
If you are not satisfied with cards from the pack, uninstall the app back to the step 1.
One reroll in Yu-Gi-Oh Duel Links will take around 10 minutes.
What to get
If you are not sure what to get, be sure to check the reroll tier list page. It will get updated from time, depending on the currently available packs, as well as the current meta.
LOL... Well, you were gonna replay tutorials and get ur starter gems... They were kinda hints that u were gonna start from ground zero again. Well, climb ur way up again, young grasshopper. Lv 13 isn't too far in.