I want to configure guest access so i am using ISE wireless setup to configure it quickly. ISE is asking for WLC shared secret. what is this "shared secret"? is this referring to snmp string? do i have to add ISE as snmp server on WLC?
That is the RADIUS shared secret.
ISE will connect to the WLC as part of this wizard and configure it with ISE as RADIUS server, so you don't need to do it manually.
As mentioned before, an entry for ISE (radius) in the WLC would be added automatically like this so you do not have to manually GUI into WLC and create it.