A grain of wheat, rice, or other cereal crop is a seed from it.

Grain is a cereal crop, especially wheat or corn, that has been harvested and is used for food or in trade.

A grain of something such as sand or salt is a tiny hard piece of it.

A grain of a quality is a very small amount of it.

The grain of a piece of wood is the direction of its fibres. You can also refer to the pattern of lines on the surface of the wood as the grain.

If you say that an idea or action goes against the grain, you mean that it is very difficult for you to accept it or do it, because it conflicts with your previous ideas, beliefs, or principles.