if you don't know what it means ... then it means:
"take the money - you are getting a good deal" ; )
it also means .... the VC thinks your company is worth $3M right now "pre money" and is proposing to invest $3M so your company would be worth $6M ("post money") and the VC owns 50% **. this is the simplest explanation.
** it does not mean you will own the other 50% of the company! This is where it gets very complicated - but at least I hope I answered your question.
CJ is dead-on.
I'll expand on the "you still don't own 50%" bit:
Even worse, you need to understand preference shares vs. common stock and how liquidation preference may affect your ownership. I'm a former entrepreneur turned VC. If you're interested in this topic I cover a lot of stuff here -- > http://www.bothsidesofthetable.com/pitching-a-vc/ also check out www.feld.com and www.venturehacks.com.