What is a valence shell?
Valence shell is the outermost shell of every element. Atom of every element have different electronic configurations based on the atomic number of each element. Electronic configuration refers to the distribution of electrons in various shells/orbits/energy levels of every atom. For example,
Electronic configuration- K L M
As you can see sodium has three shells in it. The outermost shell of sodium is called the valence shell and the electrons in the valence shell are called valence electrons.