1999 Chevy Monte Carlo muddiing

Hello,
I took my car in to get a coolant flush done, I was told by the mechanic that my car had signs of mudding, which is caused by coolant and oil mixing together, I would like to know if this is true and if not, what exactly is mudding, what causes mudding and how to prevent it form happening, and what can be done to get rid of it. Have a great day

Thanks
Mark