The Stability Pool pays out both LOAN token and PLS to the Stability Providers.
The LOAN token yield comes from a time-based emission schedule which halves in rate every year.
The PLS yield comes from liquidation gains. This happens because Stability Providers use a portion of their USDL to pay the debt of liquidated vaults. For a reward, they gain the PLS which is roughly 110% as valuable as the USDL they have lost.