Resistance (R) in the Ohm's law equation is what the electricity works against in an electrical circuit. Resistance is measured in ohms (Ω). Resistance can come in many forms, some good, some not so good. Every electrical circuit needs some form of resistance. If not, you have a short circuit, and we know what happens if we have one of those. Resistance can be as simple as a light bulb or as complex as a PCM. Both create a certain amount of resistance that is accounted for in its circuit.
A few key things can affect resistance. One of them is heat. The higher the temperature, the higher the resistance. The reason for this is that a heated substance has a lot of molecular movement, which affects electron flow. The harder it is for electrons to flow, the more resistance you have. That's pretty much the definition of resistance.
Distance can also affect resistance. The longer a wire is, the more resistance it has. Think back to our car with a tank of gas. We're going to use more gas the farther we have to travel to get back to the negative battery post. The size or amount of conductive material that the electricity passes through can also effect resistance. A wide-open path with a very conductive material has little resistance. Conversely, a small wire with a not-so-conductive material has more resistance.
The takeaway here is that resistance can be good or bad depending on where you find it. You need the resistance of the load of an electric circuit in order for the circuit to function, but if you have unwanted resistance, the circuit will not function as intended.