It ain't that hard to understand.

Lets say you are a hunter gatherer, you hunted a pig in the forest. You and your fellow cave mates had a good meal. Another cave mate arrives an hour later. Asks how many pigs you hunted, your reply 1, asks how much is left, given you had everything, and nothing is left, what answer would you give?

Congratulations you just invented zero in 100000 BC.