Cash is the amount of underlying balance owned by this aToken contract. One may query the total amount of cash currently available to this market.
SErc20 / SEther
function getCash() returns (uint)
RETURN: The quantity of underlying asset owned by the contract.
SErc20 aToken = SToken(0x3FDA...);
uint cash = aToken.getCash();
const aToken = SEther.at(0x3FDB...);
const cash = (await aToken.methods.getCash().call());