what do you mean by cross section area(electricity)?

Cross section represents a plane made by cutting across something at right angles to its length. If you cut a wire and look straight on you will see a circular plate.

The area of this cross section is called its area of cross section.

If you cut across a rectangular block, at right angle to its length, what will you see?

You will see a rectangular area. The area of this block is the area of its cross section.