The money that’s backing the Tethers. As an example - you deposit 1mm USD with Bitfinex, and they give you 1mm Tethers. Bitfinex invests your 1mm USD every night at overnight Libor, earning interest each day they do so. When someone (you?) wants to redeem the 1mm Tethers, Bitfinex gives them the 1mm USD, but keeps the interest it earned. You question is a good one, though - why would people ever pledge USD to create Tethers in the first place, given that treasuries have a higher (non-zero!) return? One example: if your 1mm USD came from a criminal enterprise, then you couldn’t deposit it at a brokerage and buy treasuries (due to KYC rules, etc).
