When it comes to flat iron, flank, hanger, and skirt steaks, the grain is apparent because it is a tougher cut of meat with less fat. These steaks have very long muscle fibers known as "the grain."
Cutting these fibers across, rather than parallel, makes for shorter muscle fibers that attribute to a more tender, and less chewy, steak.
In the photo above, see the subtle lines running through the slab of steak, and notice how the knife is cutting through those lines, rather than parallel to the lines. That's what "cutting against the grain" means!