This video helps explain the difference between a Mechanical and Digital (Electronic) doorbell kit and how to install the diode for Digital doorbell kits only:
Diodes are small items that are necessary to correctly trigger certain home door chime kits. If you are not connecting to a home doorbell kit, do not use the diode.
If your home doorbell kit is mechanical, meaning a physical striker is hitting a bell to make a "ding-dong" sound, do not use the diode as this may cause damage to your doorbell kit or transformer.
This is a mechanical doorbell kit with the cover off:
If your home doorbell kit is digital, meaning the doorbell kit is a speaker that plays tones when triggered, then you must use the Diode. See this article for more on how to correctly install the diode if you have a digital doorbell kit: