Dav files are used by some digital video recorders to store footage, but they're not exactly the most compatible files out there. That's where conversion comes in - it's like a magic trick that turns your Dav files into MP4 files that can play on almost any device.
So, what do you actually do when converting Dav files? Well, it's pretty simple: you use a conversion software that can read the Dav file and then spit out an MP4 file that's identical in terms of quality. It's like a digital copy-paste, but way more cool.