The diode is one of the basic, yet most important electronic system components. It is made by joining two materials of different charge. When a p-type (positively charged) substance is joined with an n-type (negatively charged) substance, the result is a diode, which only allows current to travel in one direction through it. The direction the current travels depends on the order in which the two substances are joined.
If the p-type substance is at the anode, and the n-type is at the cathode, we say the diode is forward biased, and that current can travel in that order (from the anode to the cathode) through it.
If it were the other way, we would say it is reversed biased, and in this case, current could not flow through it.
Because a diode only allows current to travel in one direction, it acts as a "current valve". Since a reversed biased diode allows no current through it, it can also be considered a resistor.