It means that it can switch up to a total of 15 amps. I'm assuming you are referring to an AC controller by the way. Even if you are running 16 150W 240V ropelights that only maxes out at 2400W or 10 amps. Most people are actually moving away from 240V AC lights here in aussie in favour of low voltage (typically 24V) DC lights as they are safer and the controllers are cheaper and easier to come by.