that retail investors don't play with, but it is a very active market on the institutional level.

Institutions lend their stock out for various reasons, but the most basic reason is to free up cash. If they buy a lot of stock, they have to pay for that stock on T+2 (thanks for the correction RyneK). When they lend out the stock to Party B, Party B lends them about the amount of cash that stock is worth, and there is an interest rate associated with that lending transaction. The institution can then use that borrowed cash to pay for the purchase. Make sense?

Usually that interest rate is a positive interest rate, but in the case of stocks that are heavily shorted, like BYND right now, it can be a large negative interest rate, as the supply of shares available to borrow is low relative to the demand to borrow.