You've probably heard that you should slice meat "against the grain," but why exactly? And how do you figure out where the grain is? Kenji López-Alt at Serious Eats explains it all.
The picture above should give you a good indication of how against the grain (left) looks from with the grain (right) — the one on the left is what you want. If you slice against the grain, those muscle fibres will be much shorter, meaning the meat will be much more tender and easier to chew. Slice with the grain and you'll get a much tougher piece of meat, even with all other things kept equal.
Of course, they may also be harder to cut — but a quick freeze can help with that. Check out the full post from Serious Eats for more — it goes into some serious detail behind why this happens and how to find the grain and cut it properly.