Delay (5e Variant Rule)
From D&D Wiki
You may use your reaction to delay your decisions until later in the round, effectively watching and waiting before you act. After each turn, the player may state they will take their actions, but they may not interrupt another character's turn to do so. (A player may preemptively state how many turns they would like to delay for.) A player may not delay their actions beyond the end of the round- they must either use their turn or forfeit it. You cannot take the delay action more than once in a round. Upon taking the delay reaction, a character may not take the ready or dodge actions.
This is your turn. Any effects which would trigger or take effect on your turn happen when you take this action; you are delaying your choice to act, not the turn itself. The purpose is to give characters with higher initiative more freedom of choice, as they can decide to be reactive to unpredictable changes, while slower characters cannot; they are at the mercy of higher initiative characters. By preventing characters from delaying beyond the end of the round, you cannot have a situation where everyone delays a full round just to manually rearrange the initiative order; they've already taken their turns, you're just waiting for their actions. If everyone were to delay to the end of the round, their actions would simply take place in initiative order.