If a tournament has the Check-in feature enabled, you will be required to check in before the competition begins, so that the organizer can easily know who is really participating.
You may check the status of all of the tournaments you are participating in directly in your List of Tournaments:
The status of the Check-in is available in the Tournament text box, and dynamically changes depending on the timeframe and your status within the tournament:
To check in, you simply have to click on the button and you are set to play!
And it's available on both Android and iOS versions of the Mobile App: