An open challenge is a request for a game that hasn't been accepted yet.

You can see open challenges by clicking 'Play' on the left menu and then clicking on 'Play Online' in the next window.

Then open 'Custom' dropdown, and then click on the 'Open Challenges' tab:

Click on 'list' or 'chart' to see the challenges displayed in either a list or chart format.

'Chart' (shown above) will show a graph of open games looking for opponents, and the type of game they want to play. The Icon is different depending on what kind of game it is, you can see the legend below the graph to see what each meaning is.

'List' shows the same info but in a list format like below:

Click on one of the Icons, or click 'play' on one of the list items to accept the game challenge

NOTE: clicking on the icon in the graph or clicking on 'play' in the list will accept the challenge and start the game, except for Daily games. For Daily, clicking on the icon or on 'play' will open the challenge to show further info, where you can then choose to accept or not, like below:

Here you can see the members record, whether you've played them before, their timeouts and average move speed and more.

You can use all this information to decide if you want to accept the challenge! But choose quickly, these challenges get accepted fast!

If you see the above message, this means someone else accepted the game while you were looking at it!