1) For comparison of the relative oxidizing and reducing powers of various metals: A Positive number indicates stronger reduction potential of the electrode, and it functions as a stronger oxidizing agent. A negative number means weaker reduction potential of the electrode, and hence it is a more powerful reducing agent.
The electrochemical series or the activity series is the arrangement of various electrodes in the increasing order of their standard reduction potential. The reduction potential values are seen to increase from negative to zero and positive real numbers. When arranged linearly, it would look like a number line with the positive numbers on the right side of zero and the negative numbers on the left side of zero.
An electrode when in contact with an electrolyte solution of the similar ionic nature (Example, Cu electrode in CuSO4 solution, Zn electrode in ZnSO4 solution) tends to either undergo Oxidation (loss of electrons) or reduction (gain of electrons).
Due to this oxidation or reduction, there develops a charge separation between the metal electrode and its ions in the solution creating a potential difference.