I was kind of hoping one of the other would answer this as I never use it!
BTW feel free to have asked this in your other thread as its the same guys who will answer/ignore the question here!
Since they've left you hanging I'll give it a shot:
Devmode is running the game in the developers environment. It allows you in single player to cheat! You can get the weapons without picking them up, you can be invnerable to enemy fire, have infinate ammo etc.
To launch dev mode you edit the shortcut for the game. At the very end of the file name you add the switch "-devmode" (without the quotes)
To stop using dev mode re edit the short cut removing the additional "-devmode"
I think there is a fuller description here but beware as it contains spoilers: