Now a days whats app is a very useful APP. people who have smartphone you will get whats app in almost of them. Web.whatsapp.com is the the platform from where you can operate you whats app chat or profile by using Computer.
If you want to operate your whats app account through computer then you have to go to Web.whatsapp.com from your computer there you will get a bar code you have to scan that bar code by your smart phone and after that you can operate your whats app messages from your computer.
It is just like whats app in your smart phone, but you can see all your chats and attachments in computer screen.
It is a very useful option. It is very user friendly and did not take long time to load. If you are not able to open your computer anytime then this will not help you.