u/Top_Coast2132

I'm new to electronics and I was watching video about practical uses of diodes

essentially the guy in the video said that when the external power supply (the 12v on the left) is working, then the diode is "off". But if the external power supply stops supplying voltage, the battery (on the bottom) start to supply voltage, keeping whatever your circuit is running.

My question:
what does the diode actually do here? shouldnt the battery keep supplying voltage even when the external power source is on? From what I can see, the diode is just blocking the external power supply from getting to the battery.

Here's the video I was watching with a timestamp:
https://www.youtube.com/watch?v=yPwYNUDkkZs&t=373s