Hey Felix, a seed for a trezor and ledger has 24 word seed you write down. In total the seed is 25 words long and the 25th is a standard word that you can edit to any word you want. So for example your 25th word is "car" then if someone hacks or finds your seed he has 24 words but you changed the 25th standard word to "car", so he is unable to access your trezor/ledger.
So if you wanna keep you trezor / ledger safe its recommended to change the 25th word and set a own passphrase. A password would be the pin you set on your trezor or ledger and as you can see on the trezor they can just hack it with brute forcing until they find the right pin.
An interesting thing is with passphrases you can change the 25th word to anything you want and everytime you set a different passphrase you create a totaly new trezor / ledger. So for example you can have your 24 word seed but then have 2 different passphrases as 25th word so you have basically 1 seed but 2 trezors / ledgers.
If thats too complicated for you to look up just get a ledger because since ledger uses the same hardware chips as banks use, its by now still unhackable (until someone proves it wrong ;) ). We always recommend using passphrases but if you forget the last word but have the seed you still wont be able to access your device. So be careful :)