WordPress Secret Key (Salt) are a random set of 8 variables that were added in 2.6+ version of WP in order to improve the encryption of the information stored in user's cookies.
WordPress Security Keys add an extra layer of security in your WordPress installation by encrypting sensitive data such as the username passwords by changing the plain text into a random and unpredictable encrypted password
Its essential to change your WordPress Security(Salt) Keys whenever your WordPress Site gets hacked. Even if you manage to change your WP database and user passwords if the hackers have(and they probably do)
your site Security Keys then they can easily regain access an dhack you again and again.
That said its essential to change your WordPress Security Keys if you feel like getting hacked.
*Tip: If you are too busy or too scared to deal with your WordPress hacked website the you can submit a support request with FixMyWP.com and ask for a WordPress Hacked Fix. They will probably reply within one hour or so take care of your site the same day.