If you have the seed words for Bitcoin addresses you can import them into an alternative wallet like mycelium which will restore access to your coins. I advised someone who had a malfunctioning Jaxx wallet to try importing into mycelium some time ago and it worked. You can read about it here.
https://www.reddit.com/r/BitcoinBeginners/comments/5xt8m6/my_12_word_seed_from_jaxx_isnt_showing_my_balance/
Jaxx follows BIP 32/39/44, so Copay (mobile and desktop) and Mycelium (Android, probably iOS) can restore the Bitcoin wallet from Jaxx.
^^ This worked! Thank you so much!
And the Seed phrase is that random string of words that it tells you you must remember and don't give to anyone? If I have that, then I can use any wallet?
Also that seed word phrase would also work for altcoins not just Bitcoin's right ?
It will work for altcoins if you can find a wallet for every supported altcoin that can import Jaxx seed words. I haven't used Jaxx and don't know all the altcoins it supports. There could well be some altcoins that can't import Jaxx seed words, in which case you are best backing up the private keys too.
This quote says jaxx automatically switches address every so often. If that's true, and it switches to newly generated addresses, you might be continually backing up new private keys.
https://forum.z.cash/t/restore-t-adress-jaxx-wallet-warning/8225/34
I have just looked in my wallet: I am also missing some coins. But that´s not because the change of your adress. Jaxx currently must have a problem when retrieving the blockchain, so not all coins will be displayed. You can simply enter your t-address at Zchain11 and then you can see that all your coins are still there. Hopefully, Jaxx fixes the problem quickly!
To be safe make sure you back up an altcoin addresses private key before sending coins to it.