Professional players understand many things that amateurs do not. Some of the more important concepts to grasp are:

Statistics and the mathematics involved in calculating odds

The ability to memorize or calculate probability in real time

That the distribution of cards is completely random and luck, superstition, etc do not play a factor in whether one wins or loses

Variance is huge and a very large bankroll, and the discipline to play within that bankroll, need to be adhered to

Self control is paramount, as one poor session of tilt can destroy a month’s worth of hard work and disciplined play