Double spending refers to the same sum of digital currency being spent more than once on the network, usually as part of a malicious attack or exploit. This works by making a transaction and then tricking the blockchain into stating that the transaction never took place, allowing the bad actors to spend their money once again.
Bitcoin’s Proof-of-Work consensus method is designed to prevent double-spending. To double-spend, an attacker usually needs 51% of the hashrate by running enough crypto mining equipment to take it over. With a network the size of Bitcoin, this would be prohibitively expensive even if it were possible. Bitcoin nodes are too widely distributed and there are too many other miners. However, it is possible to double spend on smaller, more centralized networks.