Given two parallel lines and a transversal, there are alternate interior angles. Remember that alternate interior angles are congruent. So, two angles on opposite and interior (meaning between the parallel lines) sides of a transversal crossing two parallel lines are congruent. This can be used to find the measure of these angles.
A fairly simple example of how we can apply alternate interior angle is this problem right here. We have two parallel lines and a transversal and I’m asking what is the value of angle a? Well these angles are in-between the two parallel lines that are intersected by the transversal and they’re on opposite sides of it, which means they have to be congruent because they are alternate interior angles, so a must be 55 degrees pretty simple, alternate interior angles always congruent.