foitsop is an acronym of first one in the series of pics, a special tag for use on flickr popularised by Mike Crowley (aka Count Bockula). Unlike most acronyms foitsop is always created using all lower-case letters.
The tag is designed to allow for easier searching of new LEGO MOCs posted on Flickr. By placing the tag foitsop on the first picture in a series of images on Flickr a search of that tag brings up only the images intended to showcase that particular creation. This reduces the number of results that need to be scanned and ensures the best (or preferred) picture is the one first seen by those searching for new creations.