Slicing meat 'across the grain' means cutting perpendicular to the direction of the muscle fibers. This shortens the fibers, making the meat more tender and easier to chew.
Meat is composed of muscle fibers that run in a particular direction, often visible as lines or striations. When you slice meat with the grain (parallel to the fibers), you end up with long, tough strands that are difficult to chew. Slicing across the grain, however, cuts these long fibers into shorter segments. This effectively shortens the muscle fibers, disrupting their structure and making the meat significantly more tender. Imagine trying to bite through a long rope versus biting through many short pieces of the same rope – the latter is much easier. To identify the grain, look closely at the surface of the meat. The grain will appear as lines running in a specific direction. Position your knife perpendicular to these lines and slice.
If you're unsure of the grain direction after cooking, try gently pulling the meat apart with your fingers. The direction in which it naturally separates reveals the grain. Always slice against this direction for optimal tenderness.