Vertical Angles are the angles opposite each other when two lines cross
In this example, a° and b° are vertical angles.
The interesting thing here is that vertical angles are equal:
a° = b°
(in fact they are congruent angles)
Have a play with them yourself (click on "vertical angles"). Notice how the 4 angles are actually two pairs of "vertical angles":
Example: Find angles a°, b° and c° below:
Because b° is vertically opposite 40°, it must also be 40°
A full circle is 360°, so that leaves 360° − 2×40° = 280°
Angles a° and c° are also vertical angles, so must be equal, which means they are 140° each.
Answer: a = 140°, b = 40° and c = 140°.