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i mean

what to study to become better chess player

Work on everything, focus on your weaknesses. 

If you are making tactical mistakes, then focus on tactics. If you are getting positionally out played, then focus on positional ideas, if you're getting blown out in the endgame, then focus on endgames. 

Each player is different, but everyone is more constrained by their weaknesses than they are helped by their strengths. 

As a general rule, below titled levels, everyone needs help on tactics, and opening theory is of limited utiltiy. That doesn't mean that you should only study tactics and never study openings. But it does give an idea for focus. 

Further, looking over many, many annotated games to get a holistic view of good play is always useful. 

To become a chess player, you just need to know how the pieces move.  On a chess website like this you could even figure that out by trial and error.  So, you don't really need to study anything to be a chess player!  Just start playing, and instantly you're a chess player.