In Play
  
For a long time, you've had a crush on your best friend's spouse. You lay awake at night, staring at the ceiling, thinking of them...sighing forlornly and imaging what it would be like to be with them. You thought you'd never have a chance. They were off the market.
Then word comes: your best friend and their spouse are getting a divorce. Soon, they'll be dating again. Finally, your chance has come (assuming you don't care too much about your best friend's feelings). Your crush is back on the market. They are...in play.
The same dynamics work for the corporate merger market. A company is said to be "in play" when it becomes a clear candidate for a takeover.
This situation can arise in a number of ways. A company can announce a merger deal, which prompts competing offers to appear. Or a company can announce that it wants to put itself up for sale, and then conduct a formal process of receiving bids. Or another company can launch a hostile bid, which again flushes other suiters out.
Whatever way it happens, the key point is that the company now has multiple bids to juggle. It's in play.