An atom is made up of a small nucleus, containing protons and neutrons, which is surrounded by orbiting electrons.
Protons are positively charged and have mass.
Neutrons have no charge (are neutral) and have a similar mass to a proton.
Electrons are negatively charged and have mass - although they are substantially smaller than protons and neutrons.
Protons and electrons have exactly the same size of charge, but opposite charge as each other.
This means, that in the first diagram above (a helium atom) there are two protons and two electrons. Therefore, there are two positive charges and two negative charges and so the positive and negative charges cancel each other out.