Poker is a game of mistakes. The less you make and the more you force your opponent to make, the more you'll profit. This is not saying that winning players are prone to not making any mistakes, they just know how to minimize it.
So, we can say that the long term winners are those players who make less mistakes than their opponents. And the long term losers are those players who make more costly mistakes than their opponents. However, roles can be reversed in the short term due to variance. A winning player an easily minimize their mistakes and still lose, while the losing player can make countless mistakes and win. However, since poker is a long term game, this sound not effect a winning player.
Mistakes include playing too many hands, playing out of position, limping in too much, cold calling raises/3bets too often, chasing draws without proper odds
, not getting value for their strong hands, playing wrong against certain play styles (trying to bluff into a call station), etc.