The picture is framed at a tilted angle. Initially used to fit tall people and situations onto wide images, but the creator don't want to put the images sideways. The technique is later used to create tension over the subject and shifting the image's focus point to further emphasize the subject(s).
The term originates in filmmaking, also known as German angle or Batman angle.
Basically if you find yourself tilting your head to one side in order to see the picture in proper perspective, this tag is appropriate.