It means just that. Site electricity supplies are somewhat crude and quite limited. If you're planning on running a TV, microwave, hairdryer and fan heater all at the same time then you're probably going to trip the supply. Sometimes this can affect other users too. You're not going to make yourself very popular if you deprive half the site of power.
If you can manage to run low power stuff from your battery(ies), so much the better. We don't use a TV when caravanning at the moment, but last weekend we managed to run a small fan heater, small fridge and a couple of lights from the mains hook up, with auxiliary lighting from the battery. I suspect we only managed to run the fan heater, though, because we were the only van on site.