This tutorial introduces and explains the concept of supernode analysis. Supernodes are a useful method to find unknown node voltages when the unknown nodes are connected with a voltage source. So solve a problem with supernodes, we need to apply KCL for the whole supernode, and also find the supernode equation (which is basically just the equation that expresses the difference between the two unknown voltages). For a problem with two original unknown node voltages, we'll have 2 unknowns, and 2 equations. If there are more unknown node voltages, then you can likely just apply KCL at them to get the remaining equations needed.