Builder's sand or Mason's sand mixed with just a tad of compost or potting soil. Not too much, mostly sand. Throw sand down on low spots being careful to keep grass blade tips exposed. Level out. Rain and or sprinkling will finish most of the leveling work. Depending on how 'low' your low spots are will depend on how long it will take to completely level. Be mindful of any weed seed that may be in the compost or sand. Not much you can do about it other than pull or spray any that may pop up.