This condition allows the automation rule's trigger to be filtered by the in-play state of the market.
This is a very simple condition, but it is a good way of highlighting how the automation conditions work.
e.g.
1) If you don't want a rule to be triggered if the market is in-play then add this condition and select 'The market is NOT in-play'.
2) If you want a rule to be triggered only if the market is in-play then add this condition and select 'The market is in-play'
3) If you don't mind whether the market is in-play or not, then don't add this condition at all. Only conditions that are added to the rule will be tested during processing; only conditions added to the rule will 'filter' the rule's trigger.