Can Someone Please Explain Offset

[WilliamsKR]

:? Can someone explain wheel offsets. I'm thinking about purchasing some 19's for my 2001 330I and I was told that I would not have any problems with mounting 19X8.5, but I would have problems mounting 19X10's. What mods are required? How will it effect my ride will the tires rub etc......

Thanks!!!!
Semper Fi
USMC "America's 911"