"Series" means one after another. Electrical components such as resistors are in series when they are connected one after another -without any junctions between them.
When adding resistors, the answer is called the "equivalent resistance." Resistors in series add up, as shown above.
Re is the equivalent resistance of the single resistor that replaces the others in series. In the animation above, Re is 12 Ω.
Video summary of the content above.
Here is the YouTube link to the above video, http://www.youtube.com/watch?v=6kdl2qLvpq4
by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)