Luck
Luck is the influence of chance, rather than skill, in a game. In poker, luck can decide the short term, but in the long run skill wins.
The simple explanation
Even the best player can lose a hand or a session because of luck. Over thousands of hands, skill becomes more important.
A closer look
Luck shows up as variance. You cannot control it, but you can manage it with bankroll rules. Good players focus on making correct decisions and accept the results. Thinking about luck too much leads to tilt.
A practical example
A player with the best hand loses to a river card. It is bad luck, not a mistake.
Full explanation
Luck is the influence of chance, rather than skill, in a game. In poker, luck can decide the short term, but in the long run skill wins.
Luck is chance, not skill.
It decides the short term.
Skill wins in the long run.
Accept variance.