- If you want to boil it down to the core  in order to explain digital cash-

You want to buy a toy you like from one of your friends. You could use some dollar bills to pay him, but that would be inconvenient as you would have to ask your parents to take you to the bank to get some money, and then you have to carry it all the way to your friend before you can have the toy. Instead you can send him the same amount of money from your computer with a cryptocurrency. It is like sending an email containing money.