Reserves are an accounting entry in each aToken contract that represents a portion of historical interest set aside as cash which can be withdrawn or transferred through the protocol's governance. A small portion of borrower interest accrues into the protocol, determined by the reserve factor.
SErc20 / SEther
function totalReserves() returns (uint)
RETURN: The total amount of reserves held in the market.
SErc20 aToken = SToken(0x3FDA...);
uint reserves = aToken.totalReserves();
const aToken = SEther.at(0x3FDB...);
const reserves = (await aToken.methods.totalReserves().call());