What is a token allowance?

A token allowance gives an ERC20 smart contract the permission to transfer a specific amount of tokens on your behalf.
This is required for a deposit or trade on a decentralized exchanged.

The allowance can be higher than your balance, these numbers don't mean you own these tokens.

Setting the allowance is done using a function called Approve.
Some 0x exchanges call this 'Enable trading' and will often set it to the maximum amount, 1.157e+59.