Why is entropy change in an irreversible adiabatic process positive? I've read this in a book

During irreversible compression, large quantity of heat is produced due to friction, while the decrease in volume is less. Thus, increase in entropy due to rise in temperature exceeds the decrease in entropy due to decrease in volume.

I don't get this at all. Is the friction they're talking about the one between pistion and container? What if we consider an ideal condition with no dissipation?
I also want to know the physical interpretation of increase in entropy in adiabatic (irreversible) process. Shouldn't it be zero? This book tried to explain it but it was confusing and I'd love to get further insight on this.
The friction they are talking about is viscous friction of the gas itself.  This comes into play when the gas is deformed rapidly, and is roughly proportional to the square of the rate of gas deformation.
Regarding entropy, the change is entropy is equal to the integral of dq/T only for a reversible process.  In an irreversible process, in addition to this exchange of entropy with the surroundings, there is also entropy generated within the system itself.