Each round has 2 open plays.
The first open play is after completing level 55.
At this point in the game you need to collect 1 of each of the sets from level 1-55 to progress to level 56.
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.
but 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 will need to collect 1 of each of the sets from level 56 to 100 to progress to the next round.
If you do choose to progress onto round 2 then you will no longer be able to play with your friends that are still in round 1.