A passphrase is optional. It's just a new word, of your choice, added to the end of your 12 or 24 word seedphrase.

A specific 12/24 word seedphrase is for a particular wallet. If you add a passphrase to the end of that, it's a completely different wallet. One person could have a seedphrase with 12 of the same words and another person could have those same 12 words + a passphrase and it would be a completely different wallet. Those two wallets would not be connected in any way whatsoever, even though their private key is almost exactly the same.

I have setup my passphrase to be very long and memorable, but easily reconfigurable (for me) so that if I want a completely new wallet, I just reorganize my passphrase, but still use the same seedphrase.

Example:

Let's say your 12 words are all the same:

goat goat goat goat goat goat goat goat goat goat goat goat

That's one wallet

Now lets create a new wallet:

goat goat goat goat goat goat goat goat goat goat goat goat + janfebmaraprmayjunjulaugsepoctnovdec [this last bit is your passphrase]

This is a completely different wallet and private key

This passphrase is similar to how I've done my own passphrase, but I use something else that is memorable to me. If I used the passphrase above and wanted to reorganize it, I could do something like:

febmaraprmayjunjulaugsepoctnovdecjan

or

decnovoctsepaugjuljunmayaprmarfebjan

and each of those would create ANOTHER completely different wallet/private key