Namaskar is a Sanskrit word. Namaskar is done to welcome and greet family,  friends, and even strangers. It is are a respectable way of acknowledging another person.  We express gratitude towards another being for their kindness and help. 

We must always start our dance with namaskar learned under the able guidance of our gurus. Namaskar may vary from one guru to another or even from one style to another. But they all mean the same i.e. asking the universe, almighty, and their gurus to bless them. A dancer also expresses gratitude towards the earth they dance on, musicians for helping them with tala and ragas. The audience for gracing the occasion with their presence. 

A dancer must always end their riyaaz, rehearsals, and dance presentation with a namaskar