Explainer: What is poison pill?

Elon Musk is attempting to take over Twitter Inc., but the company is refusing. Although he has long chafed at what he perceives as unnecessary constraints on expression on the social media platform, Musk, the chief executive officer of Tesla Inc. and the world's richest man, isn't fully clear why he is battling to buy the social media platform and take it private. The storm of financial words being flung around, with "poison pill" arguably the most prevalent, is also perplexing to many.