What is a knot in wood?

As a tree grows, lower branches sometimes die, and their bases may become overgrown and surrounded by newer layers of trunk wood, forming a type of imperfection known as a knot.

The dead branch may not be attached to the trunk wood except at its base, and can drop out after the tree has been sawn into boards. The know is often weak but can be pleasing to the sight and are valuable for that reason.