A BIP39 passphrase is an optional extra word or sentence added to your seed. The same 12 or 24 words with a different passphrase give a completely different wallet. There is no wrong passphrase: each one opens a valid, usually empty, wallet.
What it protects against
Someone who finds your seed backup cannot reach the funds without the passphrase. You can also keep a small decoy balance on the seed alone, and the real savings behind the passphrase.
What it costs
Forget the passphrase, or one letter of it, and the funds are gone: the wallet will open an empty wallet and nothing tells you why. It is case sensitive and space sensitive. Back it up separately from the seed, never together.