This is useful stuff. It's a partially fermentable sugar. This means it leaves a sweetness in the beer, as it doesn't ferment.
It also adds body character and flavour to light beers, it's supposed to help give a big head as well.
Especially useful for adding body to light beer, it may add a bit much for your taste if you add it to 3kg all malt kit.
You can add th efull 500g to a 40 pint kit but may want to use about half to start with!