Where to concentrate exactly during meditation?

In a chance I had read 'Mindfulness in Plain English' by the Buddhist monk Venerable H.Gunaratana Mahathera. - It says "After joining inhaling with exhaling,fix your mind on the point where you feel your inhaling and exhaling breath touching.Inhale and exhale as one single breath moving in and out touching or rubbing the rims of your nostrills." - I have heard about different meditation practices and I doubt that there is no such a point is drawn in Chakra Meditation. Then, - What is the purpose of concentrating on the rims of the nostrils ? - Where I have to concentrate during meditation as a beginner? - And could you explain the point , "rims of nostrills" ?