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.
Almost every recipe states to "cut steak across the grain" when preparing or serving. The grain of the steak is referring to the muscle fibers within the piece of meat. Making these fibers shorter, by cutting across or against the grain, makes the meat more tender.
More tender cuts of meat, such as rib eye, tenderloin, t-bone, and porterhouse steaks are harder to identify the grain. Make sure to take a good look at your steak before cutting into them. If you begin cutting into a steak and realize that you are cutting with the grain instead of against the grain, you can reorient your steak and begin again.