Asynchronous (async) play means playing the game without the need for both players to be online at the same time. This allows players to play the game at their own pace, taking turns when they have time to do so. This can be more convenient for players who have busy schedules or live in different time zones.
In async games, players may only take actions during their own turn. Once you end your turn, your opponent will be notified that it is their turn to play. This means if it is not your turn, you can go about your day without needing to be at your computer to respond to your opponent's actions. They will then be able to take their turn whenever they have time to do so, and once they end their turn, you'll be notified that it's your turn again.
The phases of the game are different during async play. Instead of the normal phases of the game, async phases happen in the following order:
The turn then ends in the middle of your battle phase.