In action or operation. For example, A number of conflicting forces were in play, so the outcome was uncertain. It is also put as
bring into play, meaning “to put into action,” as in The surprise witness brought new evidence into play. [Mid-1600s]
In sports, in a position to be legally or feasibly played, as in The ball is now in play. [Late 1700s]
In business, in a position for a possible corporate takeover, as in After a news item said the company was in play, the price of its stock began to rise. [Colloquial; second half of 1900s]