The call to millis(), where the result is stored in currentMillis() is like looking at your watch.
The previousMillis variable holds the time that an action last occurred.
The test, if(currentMillis - previousMillis > interval), determines if it is time to do something.
The block of code that follows (between the { and the matching, but nowhere near lined up }) defines what to do. It is in this block that you put the code to turn the two motor pins on or off, instead of the single LED pin.