The vertical angles theorem is about angles that are opposite each other.
These angles are formed when two lines cross each other as you can see in the in the following drawing.
Theorem: Vertical angles are congruent
Congruent is quite a fancy word. Put simply, it means that vertical angles are equal. For example, look at the two angles in red above.
If one of them measures 140 degrees such as the one on top, the one at the bottom is also 140 degrees.
The proof is simple. It will start with what you already know about straight lines and angles.
We will use the angle addition postulate and the substitution property of equality to arrive at the conclusion.
The angle addition postulate states that if two adjacent angles form a straight angle, then the two angles will add up to 180 degrees .
The substitution property states that if x = y, then y can replace x in any expression. Or x can replace y in any expression.