That would be the checksum. You can't throw together any 12 or 24 words you want. The final word must be computed from the rest.

It's a very complicated thing to do by hand and you don't want to trust an internet connected computer. This is why airgapped tools that can compute the checksum like nostr:npub17tyke9lkgxd98ruyeul6wt3pj3s9uxzgp9hxu5tsenjmweue6sqq4y3mgl are so valuable.