To get into a nice barrel. Either surfing or bodyboarding. Being slotted is similar to getting a barrel, but is much tighter and harder to control. Similar to how a coin slot is only big enough to fit coins, when you are slotted, the water is barrelling just enough for you to sit in quite nicely.
>>> you got slotted on that wave man... looked sick as... wasnt the biggest barrel, but fuck it looked nice n cosy in there.
>>> see that bro, got slotted hardcore on that wave... i was waiting to get dumped, but it peeled over at the last second... epic