Each round has 2 open plays.
The first open play is after completing level 55.
At this point in the game you can still collect sets from level 1-55.
When on level 56 you are still in round 1, this means that you can still play with your friends that are also in round 1.
However you will no longer receive socks from level 55 and below from the washing machine.
You can still take and receive those socks from your friends to help them progress.
The Second Open Play is after completing level 100.
At this point you can still collect pairs and sets from the whole round.
If you do choose to progress onto the next round then you will no longer be able to play with your friends that are still in the previous round(s).