The millis function returns the elapsed time since the Arduino was restarted. Think of it as "now".
To turn LEDs (or SSRs) off and on at different intervals, keep track of when the LED (or SSR) is to be turned on or off, and use millis to determine what time it is now. Then, if it's greater than the time to turn an LED (or SSR) on or off, do that. When toggling the LED (or SSR), set the next time to turn it on or off.