Leaderboard

That's credit default swaps (CDS). It's like an insurance against the default of an underlying. The underlying could be a single name company like Wework, or it could be an index of a bunch of companies. The CDS is swap with 2 legs and is quoted in "spread" but it could be seen as the fixed leg of a bond, and for the other side, it's the contingent leg which pays if there is a default in the underlying. The CDS becomes 'more' expensive if the default probability is seen as higher. Here Ackman saw that the cds spread (price) was at a low, when multiple people saw that the new virus was going to be a problem, and causing a shutdown of the economy, hence increasing the perception of a default of multiple companies. These instruments are not accessible for the rest of us. Also go watch 'The big short' if you haven't, it's interesting giving the current context. See https://pershingsquareholdings.com/wp-content/uploads/2020/03/Pershing-Square-Capital-Management-L.P.-Releases-Letter-to-Investors-March-26-2020.pdf