A salt is a long, complicated “password” that is used with secret keys to increase the security of your WordPress site. It is a best practice to put unique salts and secret keys in your wp-config.php file, and to change them every so often.
The secret keys and salts used by WordPress config look like this:
define('AUTH_KEY', '-l1z/TV0|TG>-%.8eHlCL=-9! l22XC|}V;Xn{|W}/9)38-b]E9xb^ij';
define('SECURE_AUTH_KEY', '(cJ:|y{#-J69qzRePe5tLXs<XT+d96lb:}_o-SDKk=}WO|F';
define('LOGGED_IN_KEY', 'ukWwk/6TuO/K+cE%fPDp~m.%OM&|%L6j 0g8vbQX,-[|MLCv^';
define('NONCE_KEY', 'H%Dlxp[w->|/~&=Myoj<78|qs^spRhFhyMc%6|!!F9+F');
define('AUTH_SALT', '@pQc+qa#$_ -M|;8z!|(C{;ofPa#%sLNUmeU-ea@}!Bd@qCbslA');
define('SECURE_AUTH_SALT', 'NqBaZ[J2740x~IO5Mw W+Wi)i ]C8htAxJl-jxwjtT>?~9');
define('LOGGED_IN_SALT', '+7<1,4Ta1?qtB)?b|aa$AH5i/j:IxBr,&g-R5$Z*I#=');
define('NONCE_SALT', 'Ah(!_CCH|#cfMVKLC(%Z;Jy/$3sCU2p+Bt#vuuL&=?sYI]v#V5|');
You can make up the long strings yourself, or better yet, use the WordPress Secret Key Generator. Just visit the Secret Key Generator, and copy/paste the unique keys/salts into your wp-config.php replacing either the existing keys/salt or the sample code that looks like this:
define('AUTH_KEY', 'put your unique phrase here');
define('SECURE_AUTH_KEY', 'put your unique phrase here');
define('LOGGED_IN_KEY', 'put your unique phrase here');
define('NONCE_KEY', 'put your unique phrase here');
define('AUTH_SALT', 'put your unique phrase here');
define('SECURE_AUTH_SALT', 'put your unique phrase here');
define('LOGGED_IN_SALT', 'put your unique phrase here');
define('NONCE_SALT', 'put your unique phrase here');