This is the Mechanical Engineers definition (so I stand to be corrected by any Electrical bods out there).
A diode has a high resitance when a current is passed in one direction through it and a low resistance in the other direction. So in effect it acts like an electrical one way valve.
Its used to convert the alternating current “AC” (current that flows in both directions)produced from the alternator into a direct current “DC” (current that flows in one direction).
Hope this helps
Katana