A diode is a device that allows charges to move in only one direction. A diod consists of a PN-junction, with a semiconductor N-type region (with excess of electrons) bonded with a semiconductor P-type region (excess of holes). The region closed to the junction is called 'depletion zone', where electrons from the n-region recombine with holes from the p-region, and acts as an insulating zone.
The n-region is then connected to the negative terminal of a battery, and the p-region to the positive terminal. This way, if the applied potential difference is strong enough, electrons will start to flow from the n-region (and then also from the depletion region) to the posititive terminal of the battery, while the holes will flow to the negative terminal, so a net current is flowing through the junction.
Vicceversa, if the two n-region is connected to the positive terminal and the p-region to the negative terminal, nothing happens through the junction, because both electrons and holes of the two regions are attracted by the "wrong side": in the n-region, electrons will flow to the negative terminal (so, not crossing the junction), and in the same way the holes of the p-region will flow to the positive terminal without crossing the junction, so no current is flowing through it.