The most important thing

The most important thing 

Endgames.

Sure!

Is this philosophical on the meaning of life and existence, or is this about chess because this thread is in "General Chess Discussion?"

If you mean the most important thing in chess, it would really depend on the individual player. What level are they are? What about personal strengths and weaknesses? What about natural playstyle? Personal chess goals? How much study time the person has available?

There are many nuances a chess coach would be best equipped to answer for that person, but I'd say endgames are a great place to start; so are tactics puzzles for pattern recognition; so are pawn structures (at the higher level). There are many really important aspects to chess, but "most important?" They work together.

Chess,at your level.