What is the 25th word in a mnemonic seed when used on a ledger nano s? By default ledger nano s generates a 24 word mnemonic. What is the 25th word feature? Is the the passphrase they are talking about?
From BIP39:"A user may decide to protect their mnemonic with a passphrase. If a passphrase is not present, an empty string "" is used instead."

The passphrase can be added in the transformation from the mnemonic phrase to the seed. It is not added IN the mnemonic but in an other point of the procedure.

A consequence of this is that:
"The described method also provides plausible deniability, because every passphrase generates a valid seed (and thus a deterministic wallet) but only the correct one will make the desired wallet available."

The passphrase is a second parameter in the "generate the seed from the mnemonic" procedure. If you don't provide it the procedure assumes that the second parameter is "".