The term hot describes a device with external connections, especially to the internet. A hot wallet is a Bitcoin wallet which connects to the internet. These wallets are more convenient for day-to-day spending, but are not as secure as cold storage options because they interact with the internet. Anything which interacts with the internet may be targeted by malware from the internet. For large stacks of bitcoin, it is safer to keep your wallet disconnected from the internet for as much time as possible.