I'm so sleepy right now to make sure my answer is accurate but here I go:
Double spend is to spend the same coins in 2 different transactions. It's mostly attempted when the 1st transaction doesn't have any confirmations yet, because it becomes much harder to successfully do a double spend after each confirmation.
Double spend is usually attempted when someone makes a transaction by mistake, when a transaction gets stuck because of a low fee, or of course for fraud.
There are several ways to attempt a double spend, what I know of them:
Including a higher fee in the 2nd transaction (the one you want to get it confirmed faster), asking a miner to include your transaction in the next block, these two methods work for unconfirmed transactions.
Double spend can also happen in 51% attacks or similar attacks where the attacker has high hash power. In this case the 1st transaction could have a couple of confirmations and still get reversed. Though this kind of double spend is not worth it unless the amount of transaction is so high.

Correct or wrong answer I'd be happy to receive feedback