For maximum meat tenderness, it’s critical to cut your meat “against the grain”. The grain refers to the direction in which the meat fibre runs. If you carve with the grain, the longer length of the fibre makes the slice of meat chewy. Carving across the grain shortens the length of the fibre, making it far tenderer to eat. Slicing across the grain is a technique used mostly for stir fried dishes where the meat should cook over high heat in the shortest time possible.
Think of the letter T. If the bar on top of the T shows the direction the meat fibre goes, then, position your knife to be the vertical line of the letter T. What you’re aiming for is cutting those long fibres into the shortest possible length because the shorter the fibres, the shorter the cooking time required for the beef to become tender.
Larger cuts, such as roasts, that require carving should also be carved across the grain to ensure maximum tenderness.