In the applet you see two lines (line p and line q) that are crossed by a transversal. Remember that this creates several angle relationships including: 1) Alternate Interior Angles 2) Alternate Exterior Angles 3) Co-Interior (or same-side interior) Angles and 4) Corresponding Angles.
I have measured some of these angles for you.

1) Identify a pair of Alternate Interior Angles that are measured.
2) Move the lines p and q so that they appear parallel to one another. How do the measures of the two Alternate Interior Angles compare?
3) If p and q were exactly parallel, how do you think the measure of the two alternate interior angles would relate?
4) Now also take note of a pair of measured Alternate Exterior Angles, Corresponding, and Co-Interior Angles.
5) Note the relationships between these pairs of angles when lines p and q are parallel.