Impedance is an important concept in many different realms of engineering. We often see it in our daily life, from 8-ohm speaker wire to 75-ohm coaxial cable, the right impedance is crucial to the quality of watching TV.
Simply stated, impedance describes a relationship between voltage and current. For a resistor, that is a pretty simple relationship: V = IR. For a transmission line, however, the relationship is a bit more complicated, since energy is travelling in fields between the incident and return path, usually a trace and a plane. The characteristic impedance of a transmission line must be calculated using a field solver, and serves as the basis for signal integrity analysis. For a signal, trace impedance is targeted to match the driver and receiver impedance.
For power, impedance should always be at a minimum. For direct current power delivery, that means low resistance, or as much metal as possible. For alternating current power delivery, that means very low-inductance connections to a large number and range of decoupling capacitors. This is one of the fundamental differences between signal integrity and power integrity analysis.