In poker, a slow roll usually describes a situation where a player closing the action on the river takes an unusually long time to call a bet, even though they have an extremely strong hand that they know will win the pot.
Another way in which players slow-roll their opponents is by taking a long time to turn over their hand at a showdown after their opponent has already done so, and they know for certain that their hand is the winner.
Both of these things are frowned upon by the players, and slow-rolling is never looked upon favorably. It shows disrespect for your fellow players and creates unnecessary arguments and turmoil at the tables. However, although it is a breach of poker etiquette, a slow roll isn’t against the official poker rules, and some players do it for fun or to try and put their opponents on tilt.
For example, you move all in with JTo on 9s8s7c2d3s, and your opponent takes 5 minutes before calling with AsJs, which is the best possible hand known as the nuts. This is a clear example of a slow roll since your opponent knows he has the best hand and knows he will call your bet but deliberately takes more time.