The expected return for the money that is considered (almost) risk-free.
In traditional finance, the risk-free rate is considered to be treasury note or government bond yield, although you still have some risks like the sovereignty risk.
In decentralised finance, a risk-free rate is considered to be an US dollar lending pool rate, like one you would get from Aave USDC pool.
See also
Stablecoin
Lending pool
Passive strategy
Drawdown