The electrochemical series is a series of arrangement of metals according to the order of the tendency of the metal to lose electrons to form positive ions.
Elements which placed higher in the electrochemical series

The metal ions are weak oxidising agents because they do not have a tendency to gain electrons.

In a reaction of displacement of metal, a metal which is placed on the top of the electrochemical series (more electropositive) can displace other metals that lie below it ( less electropositive) from its salt solution.
So,

there is a transfer of electrons from a more electropositive metal to the ions of a metal which is less electropositive.
The more electropositive metal acts as a reducing agent. The metal experiences oxidation and is oxidised to a metal ion.
The metal ion which is less electropositive acts as an oxidising agent. This ion experiences reduction and is reduced to a metal.

Examples
The reaction between magnesium and copper(II) sulphate solution

Mg + CuSO₄ → MgSO₄ + Cu

The blue colour of copper(II) sulphate solution turn colourless.

Mg → Mg²⁺ + 2e

Cu²⁺ + 2e → Cu

Mg + Cu²⁺→Mg²⁺ + Cu

Note:

Oxidising agent: Copper(II) ion
Reducing agent: Magnesium

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