"The three main functions of a currency are as a unit of account, a medium of exchange and a store of value." What does it mean? How does it makes sense to trea

"The three main functions of a currency are as a unit of account, a medium of exchange and a store of value." What does it mean? How does it makes sense to treat e.g. the two last properties (that money is passed between people and that money symbolizes value) separately?