What it means
A dub is a win, a victory, a successful result: that's a dub, we took the dub. The upbeat opposite of taking the L, born from the spoken letter W for win.
Usage examples
"Closed the deal on the first try, that is a dub."
"We took the dub in the final round."
"Got the refund and kept the discount too, that's a dub."
"Ngl, making it through that exam on two hours of sleep was a dub."
Where it comes from
It comes from saying the letter W out loud, which sounds like dub in fast casual speech. Since W already meant win, people started using dub as a noun for a victory or anything that went right. It spread hard through sports, gaming, and online talk.