It means updating a devices firmware, the software which it uses to perform its functions.
This is usually done to take advantage of new features that have been developed. In RC it’s commonly used in flight controllers in UAV’s, transmitters, ESC’s and modules. All these things have some form of ROM memory to enable the new program to be stored.
Either that or I have been flashing in public entirely incorrectly all this time...
Yes, if you get it wrong the device won’t work. Generally you won’t need to change firmware to fly stuff, especially if you stick to more main stream electronics and radios, which like all devices from big manufacturers are designed to give the minimum of tech headaches to users. I have a £50 TX and several planes, a dozen or so ESC’s, not needed to change any firmware.
OpenTX is the most common radio type where you might change the firmware but big updates are not super often, plus you can back the whole lot up first, just like you would with anything else.
Yes, if you flash the wrong version or perform the procedure incorrectly you can "brick" your device, rendering it inoperable. You definitely want to be fully aware of what you're doing when you flash firmware!
Is there a particular device that you're wondering about?
Not really Kendalf, I have been considering the FrSkY X9D , and in the course of my reading some of these posts, I've come up against this phrase, and wondered what it was all about. I'd probably get some to do it for me, if needed anytime, as I suffer from natural tremors which means my hands shake a lot. How I manage to solder at all is a wonder to me.
Flashing OpenTX is actually very easy, there are a bunch of tutorials on YouTube that walk you through it. Unless you need the bleeding edge features, you should be fine with whatever version it comes with though.