Remember, current is the flow of electrons. Electrons (typically) flow through a wire or trace. A hydraulic analog would be water flowing though a pipe. In order to "push" the electrons along a wire we need an energy source. Figure 3-1a shows a battery as the energy source. Similarly, water needs a force to push it through the pipe. Figure 3-1b shows a pump as the source of the force pushing water through a pipe.
Figure 3-1. Resistance can be modeled by a crimp in a hydraulic pipe.
A resistor affects the flow of the electrons. (Of course, so do capacitors and inductors.) Its effect is to impede (related to impedance) the flow. The analog would be a crimp in the pipe that impedes the flow of water. A small crimp (resistance) offers a little impedance to the flow, whereas a bigger crimp offers a lot of impedance. A very large crimp (resistance) might almost stop the flow altogether.
A small crimp might allow a certain flow of water. A larger crimp would reduce the flow and cause pressure to build up behind the crimp, effectively fighting the pump pressure. If we wanted to maintain the same flow with the tighter crimp (bigger resistance) we would have to provide more pump force (voltage) to overcome the pressure backing up behind the crimp. Thus, there is a relationship between the size of the crimp (resistance), the flow of the water (current), and the pump force (voltage).