Initially the coins are given to raise a help request if you are stuck at some question,
the request will be sent to the users who have already solved that question and 10 coins are deducted from your 50 coins for each request raised.
How can you earn coins ?
You can easily earn coins by helping someone, who is stuck at a problem which you have solved before by answering them.
Number of coins that can be earned, depends on the difficulty of the question.