If you visualize this, or draw a quick picture, or think about perimeter more generally, you might know that perimeter is all of the sides of a shape added up. For a rectangle, you have two longer sides, the length, and two shorter sides, the width. So to find the perimeter you do length + length + width + width. You have four sides, and 2 sets of sides that are of equal length (rules of being a rectangle).
The 2 in 2(l + w) probably just represents that idea, that when you add one "length" side and one "width" side of a rectangle, you can double it to get the perimeter.