Playing poker is quite simple if you are not pretending to smth elevated, but to become professional, not only skills matter but also personal characteristics, and this is very hard to teach
I think everything is based on your techniques that you use when you are playing to receive your opponents so they will not be able to understand your present mood. Professional poker players have different strategies they can put in place to confuse there opponents not to be able to read there mind. Experience also matters a lot so we have to know that experience is the best teacher.
^ That is bluffing.
Bluffing sometimes made your lucky day but not all the time, if you have opponents that have the same technique used, it will probably not work because both of you pretending about the emotion. But I don't know how it will work in online poker games if your opponent is online, there is no bluffing and your luck depend on the cards that you get. I don't know how effective poker game online courses are right while it is free over the internet, I even watch a different tutorial on Youtube and try my skills using an app that is playing poker for free.
Not only bluffing. Your opponent can analyze your bets and try to understand your hand. And can change strategy to confuse him. And in online poker we have some other rules - we don`t see the opponent, we can`t read his emotions. But i agree that there free courses and it`s enough for the most players.