A Hot Wallet is a cryptocurrency wallet that is connected to the Internet.
Wallets can be classified as either hot or cold.
Hot simply means that a wallet is connected to the Internet, perhaps because it’s a wallet at a centralized cryptocurrency exchange or a wallet offered as a web browser extension on your desktop computer, like MetaMask.
Both types of wallets are always connected to the Internet.
A cold wallet, like a piece of paper with your private key or seed phrase copied to it or a USB thumb drive, is not connected to the Internet.
Being connected to the Internet inherently makes a hot wallet less safe than a cold wallet because the Internet potentially exposes a hot wallet to hackers and blockchain exploits.
See Wallet for more information.