Law of conservation of charge says that the net charge of an isolated system will always remain constant. This means that any system that is not exchanging mass or energy with its surroundings will never have a different total charge at any two times. For example, if two objects in an isolated system have a net charge of zero, and one object exchanges one million electrons to the other, the object with the excess electrons will be negatively charged and the object with the reduced number of electrons will have a positive charge of the same magnitude. The total charge of the system has not and will never change.
This concept is important for all nuclear reactions—alpha decay, beta decay, gamma decay, etc.— because it allows scientists to predict the composition of the final product in the reaction, shown in Figure 1.
Charged particles are allowed to be created or destroyed, as long as the net charge before and after the creation/destruction stays the same. Therefore this must happen with oppositely charged pairs of matter and anti-matter.