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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

I can see how Taleb, by either using far OTM SPX puts close to expiration or VXX calls in a similar way, could have generated 3600%. I backtested SPX using -4 Delta 2-4 DTE puts from 2/21 to 2/28, rolling 30 minutes before the market close each day back into -4 Delta/2-4 DTE puts and trying to limit the trade to no more than 10% of the available open interest (therefore, requiring multiple strikes near 4 Delta & different expirations). Doing so generated 4,000%. However, if left on until the next trading day, the return dropped all the way down to 83%. So, getting in and out at the right time was critical. However, I have no idea how Ackman went from $27 million to $2.6 billion. In the article link it says: Pershing Square used credit protection on investment-grade and high-yield bond indexes to land the massive profits. The assets rise in value as the odds of corporate defaults increase. What could he have used that jumped about 10,000% that had enough volume to accommodate such large amounts of capital? By the way, in case you might be interested, that one week backtest if rolled each time the -4 Delta put reached -10 Delta intraday would have generated over 12,000% (but ending up at -13% if left on until the next trading day).