Contact to customer conversion rate is used to assess how well you convert a list of contacts into customers. If you have a contact list of 10,000 and you convert 1750 of them into customers your conversion rate would be 17.5%.
The higher the contact to customer conversion rate the more sales that are being made. You can have a large contact list and not get any customers if you have a problem converting. To improve this percentage you can try different methods of converting contacts to customers and see which works best.