Dig around your kit a little more, and you'll find some new and exciting components with long leads attached to them.
These are diodes—circuitry's "one way" signs. Their job is to make sure that current only travels in the forward direction, from anode to cathode, while blocking it from traveling in reverse. Current enters the anode (positive terminal) and leaves the cathode (negative terminal).
Since they only allow current to travel in one direction, it is rather pertinent that you install diodes with the correct polarity. The stripe on a diode signifies which side is the cathode.