A diode passes current in one direction, and blockes it in the opposite direction. It is rated for voltage and amperage, for example 1.5 amp @ 50 volts.
The amp rating is the amount of current it can pass without breakdown
The voltage rating is amount of reverse voltage it can withstand without break down.
In this application for alternator 1.5A @ 50 volt would be minimum requirement, higher amp and volt rating give more reserve protection and are fine to use. They are easy to use and can be installed anywhere between switch output and #1 pin on alternator plug. Marked band goes toward alternator.