ok so if you sell a CDS for 100bp and then the market moves to 90bp, you have a profit of 10bp.   But how much is that actually worth in dollar terms?  Suppose you then buy the CDS for 90bp, what have you got?  You have 10bp per annum until the reference entity defaults, which is worth 10bp * the Risky pv01 of the contract.   Hope that explains it.

The risky pv01 is the value of a 1bp annuity paid by the reference entity.  It is 'risky' because if the reference entity defaults, it stops.