An initial public offering with tons of demand from buyers of that security.
Investment banks regularly underprice IPOs, in part as a risk (to them) reduction strategy, and to establish feelings of love from the buyside investors whom the bank wants to embrace the offering.
A "hot" IPO is one that is over-subscribed by a big multiple, like 5x, 10x, 20x. Those hot IPOs typically zoom upward the first day of trading, coming public at, say, $15 a share and seeing their first print at $34 and finishing the week at $50. Or something like that.
The initial buyers at $15 made a minor killing. Free money, when its yours to be made, is a good thing.