The horizontal beams are what I like to think of as "rail road tracks". The metal portion of the tracks anyway. Notice how angle 2 and angle 8 are inside the tracks, so we consider them "interior angles"
Furthermore, they are on opposite or alternate sides of the transversal line. The transversal line is the non-parallel line that cuts through the "rail road tracks".
Put those two facts together and we get "alternate interior angles" as our final answer
