Unit of account - When you walk into a restaurant, the menu tells you that a hamburger costs $5 and a steak costs $15. You know what this means and are able to compare these prices. If, on the other hand, apples and oranges were used as units of account, comparison between the costs of goods and services would be much more difficult.
Medium of exchange - When you hand the waiter a five-dollar bill in exchange for your hamburger, you are using money as a medium of exchange.
Store of value - When you work, you are paid a wage. The portion of that wage that you do not spend gets saved. By saving money, you are able to spend some of it now and some later.