The electrochemical series is built up by arranging various redox equilibria in order of their standard electrode potentials (redox potentials). The most negative E° values are placed at the top of the electrochemical series, and the most positive at the bottom.
The electrochemical series
equilibrium E° (volts)
Remember that in terms of electrons:
Oxidation is loss Reduction is gain
Reducing agents and oxidising agents
A reducing agent reduces something else. That must mean that it gives electrons to it.
Magnesium is good at giving away electrons to form its ions. Magnesium must be a good reducing agent.
An oxidising agent oxidises something else. That must mean that it takes electrons from it.
Copper doesn't form its ions very readily, and its ions easily pick up electrons from somewhere to revert to metallic copper. Copper(II) ions must be good oxidising agents.