Simply put, protons and electrons cannot be created or destroyed. Since protons and electrons are the carriers of positive and negative charges, and they cannot be created or destroyed, electric charges cannot be created or destroyed. In other words, they are conserved. One way to think about conserved properties is that the total number of protons and electrons in the universe is constant (see Note below).
Conservation is a common theme in chemistry and physics. When you balance chemical equations, you are ensuring that the total number of atoms remain constant throughout the reaction. Here, it is the conservation of mass that is concerned. Another common conservation principle is energy. We usually use this principle in physics when we equate the initial energy of an event to the final energy of an event. If a baseball is thrown upwards at an initial kinetic energy, #E_k#, the gravitational potential energy, #E_"PE"#, will be equal to #E_k#.
To give a brief quantitative overview of electric charge, the unit for charge is the Coulomb, denoted by "C". A proton has a charge of #+1.602*10^-19# and an electron has a charge of #-1.602*10^-19#. These are referred to as the elementary charge.
Note: While it is a good model to think of conservation as an inability to increase or decrease the total number of protons and electrons, it technically isn't 100% accurate. Under some circumstances, protons and electrons can be converted to other particles in certain nuclear reactions, but in doing so, the net charge for the reactions is zero.