[donavanm]

FYI "convertible" instruments aren't new, I suspect its just unusual phrasing that's tripping up your search. "Convertible bonds" or "convertible securities" are more common terms IME. Either way it's typically a bond/security that can be held _or_ converted at a preset strike price. Eg a convertible bond has a slightly discounted coupon (interest) for the holder. The bond can be converted to a fixed number of shares (equity), effectively at at a pre defined price/share. If the per-share price appreciates the bond holder can convert at capture the vallue above their strike price. If the associated equity falls (or doesnt appreciate) the bond holder 1) holds to maturity 2) is senior to equity holders for recovery.

In short, the allbirds financiers are taking a slightly reduced interest payment in exchange for the option to capture stock price appreciation if the gamble finds a greater fool.