@Valkiri. Transistors are very useful and powerful little devices. Essentially they let you control the current flowing from one place to another by using a third and unrelated current source. For instance a pulse of current from the arduino can be used to allow current to flow between two points in a separate toy circuit. Its hard to go into much detail in a short answer. I suggest you look up transistor switching. its one of the most basic applications of the transistor and is easy to get working with very few parts.