RonaldoPosdef has explained in detail what an inverted winger is. However let me get into a bit more detail.
Soccer players no matter how good they are always rated by their goal-scoring ability. So there are a lot of wingers who are amazing goalscorers but will not play as total forwards as being in midfield will allow them to get more of the bestball. Ex Messi, Ronaldo, Ribery, Robben. And if you see, all these players will always play as inverted wingerwingers.
Ronaldo perhaps is the Best amongst all of these as he doesnt seem to have a weaker foot but he is not ambidextrous, so his right foot is his stronger foot and hence he tends to play on the left side of midfield so he can cut in and score goals.