What is a Poison Pill (or Shareholder Rights Plan)? [legal concept - Twitter used against Elon Musk]

With Elon Musk's attempt to acquire the entirety of Twitter and the Twitter's Board of Directors seeking to counter this acquisition by instituting a poison pill, a.k.a. a shareholder rights plan, to challenge what they perceive as a hostile takeover, we look at the legal concept of the Poison Pill.
Poison Pills, which are effectuated by means of a shareholder rights plan, is designed to enable the existing board of directors to fend off a hostile bidder from taking over the company on terms that the board of directors does not believe best advance the interests of the shareholders, although it is also very much impacted by the board members' own personal interests in not relinquishing their position and control over the company.
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