The top part, on the forehead, symbolises the footprint of the Lord. On the top of the nose, that symbolises or looks like a tulasi leaf. Both are very important and very auspicious. In India, all the temples of Visnu or Krishna, the main domes or sometimes the walls of the temples, you will see it has large scale tilak marks. These are the traditional markings for a temple of the Lord. Krishna, the Lord, is present in our hearts in other words our bodies are temples of the Lord. Therefore we decorate them as temples of the Lord. That is the significance.