This is a law that deals with basic facets of application of conservation of heat. These laws are Law on Conservation of Charge and Law on Conservation of Energy. In case of circuits that are complex, this law holds ground. The basic factor that is taken into this case is that amount of total current that is introduced in a concerned juncture should be equal to that amount of current that leaves that juncture point.
Also, while dealing with Mesh Law, it can be seen that rise of potential that is present within a loop is equivalent to summation of potential within that loop. However, a hypothetical situation of current flow is to be taken in this case.