Hot Wallet vs Cold Wallet
A hot wallet is connected to the internet; a cold wallet keeps keys entirely offline.
In depth
Hot wallets (phone and browser wallets, exchange balances) are convenient for frequent trading but exposed to malware and phishing. Cold wallets (hardware devices, paper backups) sign transactions offline, so an attacker would need physical access. Most people keep a small hot balance for activity and the bulk in cold storage.
Why it matters
Exchanges — including StableX — use this split to protect customer funds; you should too.