Onsite warranty means when ever you have a problem with the device which u have purchased and is given onsite warranty, the respective company will send its technician to your home or workplace to repair the product. The technician will try his level best for repairing the product at home itself. If it is not repairable then he takes the product to the service center or lab for repairing.
Basically Onsite means "Service at your door Step". There is no need for you to take the products to the authorized service center for repairs. Mostly air conditioners, Refrigerator, PC's, Washing Machine and other electronic goods which are difficult to take to the service centers are given Onsite warranty.