NPK stands for "Nitrogen, Phosphorus, and Potassium," which are the three nutrients that compose complete fertilisers. Nitrogen is a major part of chlorophyll and the green colour of your lawn. Phosphorus and Potassium play key roles in the growth of roots. You should use a starter fertiliser with an NPK ratio of around 5:6:6 for new lawns to help develop their roots. Once established, you should apply a complete fertiliser in Spring, Summer and Autumn. An NPK ration of 22:3:4 would be suitable.
Overfeeding your lawn with excessive amounts of fertiliser may result in a toxic build-up of too many nutrients and can result in your lawn being quite sick. Furthermore, overuse of fertilisers creates excessive growth rates in the green leaves and thatch which will require more frequent mowing and scalped areas may turn brown. Your lawn can absorb only so much fertiliser at once then it cannot take any more. At this point, excess may be washed into the surrounding environment. Also, over applying fertiliser can easily result in your lawn being badly burnt and damaged from both the excess fertiliser and hot weather.