A dirty flash is when you already flashed a ROM, let's say Cyanogenmod 11 (KitKat), and then without doing a data/factory reset in the recovery, you flash a newer ROM on it (let's say Cyanogenmod 12 (Lolipop)). SOMETIMES this can work, but 95% of the time it won't without glitches.
This won't kill your device, merely a chance for a softbrick. When that happens, you just go to the bootloader > recovery and then data/factory reset, and you install the newer ROM.
You will lose your apps and data, though, that's why some people try a dirty flash first.