Postulate 5

That, if a straight line falling on two straight lines makes the interior angles on the same side less than two right angles, the two straight lines, if produced indefinitely, meet on that side on which are the angles less than the two right angles. It might be easier to understand the opposite idea described here. If two lines are perpendicular to a third line, the original lines are parallel. So this postulate is saying if two lines intersect a third line

When the lines are parallel, the two indicated angles must sum to 180

Since the lines are not parallel, the 2 indicated angles cannot be 180