what does "flash the file" mean?

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1

I am trying to follow directions on this site https://www.pidflight.com/pidflight-lap/build-your-own/

I've built it all just fine.
but one of the steps is download file and flash it to arduino.
it is called a firmware 2.2.0 and it is a .hex file.

what does flash the file mean?

Ive watched 3 videos about building this thing and they all just say "flash it" as if I'm supposed to know what this means.

Google leads me to believe it has something called a bootloader, but I'm not sure what that means.
I assume it isn't as simple as copy and paste the .hex file on arduino nano is it?

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“Flash the file” means the same as “upload” in the arduino ide. I don’t recall whether the ide knows how to upload “foreign” .hex files. If not, you might have to learn to use avrdude.