Understanding electrons is important because they determine a compounds chemical properties. Electrons are arranged around the nucleus in energy shells. The most important energy shell is the outermost one, known as the valence shell. Valence electrons are the first to come in contact with and interact with other atoms.
The number of valence electrons increases sequentially as we move across a period on the periodic table.
Atoms are most stable when they have an octet (eight) electrons in their valence shell. They can either gain electrons to fill up their valence shell or lose electrons so that they expose the full shell underneath. Either of these actions would constitute a chemical reaction.
Noble gases are so nonreactive because they already have a complete octet in their valence shell.
There are some exceptions to the octet rule. Hydrogen and Helium are satisfied with 2 electrons in their valence shell.