From the website:

Welcome to SwarmSketch: Collective sketching of the collective consciousness.
SwarmSketch is an ongoing online canvas that explores the possibilities of distributed design by the masses. Each week it randomly chooses a popular search term which becomes the sketch subject for the week. In this way, the collective is sketching what the collective thought was important each week. (Due to increased traffic sketches are currently being updated after about 1000 lines)
Each user can contribute a small amount of line per visit, then they are given the opportunity to vote on the opacity of lines submitted by other users. By voting, users moderate the input of other users, judging the quality of each line. The darkness of each line is the average of all its previous votes.