The way you can think about it is by asking this question: how many times does a unit square fit inside a rectangle? That is literally the definition of a surface area.

Now let's try to use an example. You have a rectangle which is 7.38 units wide and 3.61 units tall. How many times can a unit square fit inside that shape?

The side of that rectangle is 7.38 so what we are going to do is place seven unit squares side by side in a row. Then add another 0.38 of a square. Now we have created a rectangle with an area of 7.38 because we just put 7.38 unit squares side by side.

The next thing that we will do is stack those rectangles that we just made (surface area of 7.38) on top of each other. How many you ask? First we will stack three of them. Then, you guessed it: add another 0.61 of a rectangle and what we have now is a rectangle with an area of 7.38 x 3.61. See how the area of that unit square got first multiplied by 7.38 and then by 3.61 to create that original rectangle that you had, with the surface area of 7.38 x 3.61. That is how many unit squares fit inside your rectangle. It works the same way with the same logic for any rectangle with any side lengths. You just start asking how many times you put the unit squares side by side, and then you ask how many times those things go on top of each other.