Blowby?

[ticopowell]

So my wife doesn't like my truck, but she said that if I trade it in that I can get a new ecoboost, So we went to the local stealership to see what they could do(I didnt expect much), so they looked at it and said that because of blowby in the engine they can't give me very much for it. What does that mean? The salesman had no idea and couldn't even start to explain what it meant. all he said was that smoke came out when they opened the oil cap.
Can anyone explain this to me, and what I can do to fix it? the truck seems to run just fine to me so I can't think that it is too bad of thing so far, but I assume that it can get worse and cause some type of damage. let me know, thanks.
Caleb