What should be the “Secret” in JWT?

1. Does this Secret has to be unique?

It should be unique to your application — it needs to be a secret, after all — but it won’t be unique for each token. Rather, you should have a relatively small number of secret keys at any given time (e.g., usually having just one key, but having brief periods where you have two keys as you rotate from one to the next).

2. Shall I use the hashed version of user’s password for secret?

No, for two reasons:

1. Suppose that your user has a relatively insecure password, like GoPackers123. Using the password in your secret then means that someone can easily test a given potential password to see if it results in the right signature; and, more to the point, they can easily test huge numbers of potential passwords to see if any of them gives the right signature. This is an offline attack, so you would never even know it happened.
2. This would require you to distribute all of your users’ password hashes to every system that needs to hold the secret. If you have more than a trivial number of users, this can become a pretty serious burden on your secret-distribution mechanism.