It refers to blocking of ads appearing on the web. This mainly refers to graphical ads on the web

The concept was triggered thanks to adopters. The application found its growth with its integration with other applications developed by companies like Symantec and McAfee.Few online users believe that by eliminating ads and banners that are slow to load, downloading speed can be quickened. However others believe that by doing so, publishers could be put too high risk as their revenues come largely from online ads. Also, blocking ads can have definite consequences on web surfers round the world.