Lighting schedules. 12/12 = 12 hours light, 12 hours dark - used to flower. 18/6 = 18 hours light, 6 hours dark - used to veg 24/0 = 24 hour light - used to veg 24/7 = 24 hour light
By reading all the sticky threads at the top of each forum. Sorry if that sounds like I can't be arsed to answer your simple question, but if I gave you the answer to that, you'd be asking a whole heap of other simple questions soon after, the answers to all of which are contained in the stickies. For your own sake; read them.