A crypto passphrase is a series of common English words, typically twelve or sixteen, which lets you generate and recover a digital wallet. Your passphrase is the seed to recover your public and private keys, your wallet's key pair. (Here's how to create a passphrase with the Solana command-line interface.)
With your public key, people can send you money and see your transactions (to check). With your private key, they can move your funds.
It's crucial that when you install or recover your wallet from your passphrase in wallets such as Exodus or MetaMask, you should protect it with a strong password. That password will let you unlock the wallet on that device. But as that password has nothing to do with your keypair, you can set a different password on each device, which will disappear if you remove the wallet from your device and recover your wallet from your passphrase.