From the website's description:
#Nofilter is one of the most popular hashtags on Instagram. Using it is a way of telling people that you haven't used any of Instagram's built in filters to enhance the photo. But with over 40 million photos uploaded on Instagram daily, there are obviously quite a few cheaters out there. People that want you to think their photos just turned out that great, completely without the help of any filters. And you had to believe them - until now.