Took a bit of digging but I found the SEC filing [1] and the press release [2]. I'll quote from the press release because it is more easily understood. I condensed it a bit but the gist should be clear.

> In general terms, it works by imposing a significant penalty upon any person or group that acquires 15 percent or more of the shares of Common Stock without the approval of the Board.

> the rights will become exercisable if an entity, person or group acquires beneficial ownership of 15% or more... in a transaction not approved by the Board

> each right will entitle its holder (other than the person... triggering the Rights Plan, whose rights will become void...) to purchase.... additional shares of common stock

The SEC filing is a bit dense but then if you scroll down to a bit you get

> entitle the holder thereof to purchase, for the Exercise Price, a number of shares of common stock of the person engaging in the transaction having a then-current market value of twice the Exercise Price.

[1] https://www.sec.gov/Archives/edgar/data/0001418091/000119312... [2] https://www.prnewswire.com/news-releases/twitter-adopts-limi...