[umanwizard]

Imagine you have 100 square tiles that are each 1cm x 1cm. You can make 20 groups of 5 tiles each with them — this is what it means to say 20x5=100.

Now take each of your groups and arrange its 5 tiles into a vertical line. Each line is now 5cm long and 1cm wide.

Arrange the lines side by side. You now have a rectangle whose height is 5cm and whose width is 20cm. You already know its area 100 cm^2, because you made it out of 100 tiles that were 1 cm^2 each. And now you can see that its area also corresponds to the multiple of its side lengths.