A quint is five identical tiles in one group. You will not find it in Chinese or Japanese mahjong, and the reason is arithmetic: a standard set holds four copies of each tile. Five of a kind is impossible unless something can stand in for the fifth, and in the American game that something is the joker.
Why it only works here
American mahjong is played with 152 tiles, which is 144 standard tiles plus eight jokers. Those eight jokers are what put a quint within reach. Hold four 5-dots and add a joker, and you have five.
If you are holding only three of a kind, two jokers get you there. A quint always needs at least one joker, because there are never more than four real copies of any tile in the set.
Where you will see one
A quint is not something you decide to build on a whim. Like every winning hand in this game, it has to be printed on the current year's card. When the card wants a quint, the line says so, and it also tells you whether jokers are allowed in that line at all. Some hands forbid them outright.
So read the line before you commit tiles to it. Players who skip that step find out halfway through a hand they can no longer win, which is the worst possible time to learn it.
How it differs from a kong
A kong is four identical tiles, and it triggers a replacement draw from the dead wall. A quint is five. Do not assume the kong procedure carries over exactly: the tile count and draw details follow the card and the League's written rules, so check them rather than guessing.
Count your tiles as you go either way. A hand built around an oversized group is exactly where a quiet miscount turns into a dead hand, and tile count is the first thing anyone at the table checks.
What a quint really costs
Every joker you spend on a quint is a joker you cannot spend anywhere else, and there are only eight in the set. That is the real price. A quint can be the most efficient use of a joker on your rack or the most expensive one, and the difference is whether the line you are building toward is a line you can actually finish.