A discard pile is a special pile of cards, turned face up, placed next to the stock.
In a game with a discard pile, typically you have to draw a card from the stock or from the discard pile at the start of each turn. And at the end of the turn you must play one of the cards in your hand to the discard pile, where it becomes available to the next player.
It is never allowed to play a card to the discard pile that came from the discard pile in the same turn. (But if you drew a card from the stock, then it is allowed to play it to the discard pile immediately.)
Games with this rule
In Rummi, the games Jin Rummi and Tonic Rummi have a discard pile.
In general, some Rummy games like Gin Rummy or Tonk have a discard pile; orthers, including Rummikub®, don't use a discard pile.