We use money all the time. What is it? A resource? a Technology? Is money just pieces of paper? Does it have objective value? Is it a social construct?
Money is a medium of exchange:
Barter trade is a very arduous and time consuming method of exchange. It cannot scale to a wealthy modern economy. A money can however step in between goods and makes trade vastly more efficient. It negates the need to find individuals who coincidentally have goods you want and who also want your goods. Money also provides flexibility in timing of exchange. It is unlikely that your production and consumption are very well aligned.
One property of money that facilitates its use as a medium of exchange is fungability. This essentially means that all units are indistinguishable and are therefore interchangeable. Without fungability, transactions can not be made easily. Each unit of money would have to be compared and valued and the asset could not become a money because it would be inefficient and therefore not become widely accepted (diamonds vs gold). Becoming widely accepted is a key component to an asset becoming money. Money is a network and the larger the network the more valuable it is.
Money is a store of value
Hard money holds its value over time. This is most directly due to its limited and preferably predictable supply. If the supply of money could be greatly increased, this would decrease the value of each individual unit of that money. People are less willing to accept something that will lose value.
Money is a unit of account
Austrian economics teaches that value is subjective. There is no good or service out there that has some intrinsic or objective value, but value is given to things through human action. Through ongoing exchange, goods are measured against a monetary unit. Money becomes the metric, although prices are continuously changing. Money gives us prices that reflect exchange ratios between good and the continual discovery of those prices is what gives entrepreneurs and consumers information on how resources should be allocated, saved and consumed.