When carpenters sand a piece of wood, they "go with the grain." Grain is the texture on piece of wood, which often forms long patterns that show the direction in which the tree grew.
You may have heard someone say "there's a grain of truth to that" when referring to an outrageous story that has a tiny bit of truth to it. A grain can also be a speck or a particle — the tiniest bit of something. Don't confuse this with another definition of grain, the kind you find in your bread — wheat, oats, alfalfa, and so on. Check out all the grains in the ten-grain bread next time you're in the bakery!