Explain what is meant by the terms grain and grain boundary?

If you look at wood you can see that the cells all lie in a common direction. The wood is strongest in that direction, and is easily split or broken by a force applied across the grain. Diamond has a grain and in one direction it is so hard that it can't be scratched even by another diamond. But if you scratch glass with the point of a brilliant cut diamond, you run the risk of breaking it.

The same is true in metals but harder to see. Various crystals of metal* and other ingredients form at specific temperatures and that determines a lot of characteristics, all generally called "temper". A grain boundary is where two grains meet and is considered a defect in the crystal structure.

*Primarily iron, but silicon is also of interest.