Lactose is commonly known as milk sugar. it is non fermentable and is used to maintain a natural sweetness in your brew. it can be used for beer, cider and wine making, though maltodextrin is probably more widely used these days.
It is an important part of a milk stout recipe. try 100-150gm for a 23 Litre brew. Preferably add at the start of fermentation as it needs some time in the brew to blend in flavour-wise.