One square meter is how much meter?

A metre is a distance - a square metre is an area.

A square metre is the area occupied by a square which is 1 metre long on all sides.

It is also the area occupied by any rectangle whose sides, when multiplied together, give a figure of 10,000 square centimetres... so a rectangle with sides of 50 centimetres and 200 centimetres, or a rectangle with sides of 80 centimetres and 125 centimetres.... and so on.