A mineable digital coin (cryptocurrency) is a coin that is created and can be acquired through the mining process.

What does it mean if a cryptocurrency is mineable?

By mining, you can earn cryptocurrency without having to put down money for it. Bitcoin miners receive Bitcoin as a reward for completing "blocks" of verified transactions, which are added to the blockchain.

What does it mean if a coin is not mineable?

Non-mineable cryptocurrencies are not designed to replace fiat money. They're digital assets issued usually during an ICO (initial coin offering). The more promising the ICO and its goals, the better chances for a non-mineable cryptocurrency to increase in value upon completion.