At its heart, it means taking a detrimental trait or sacrificing a comparably useless stat (minning) in exchange for buffing another stat (maxing). Nowadays, people just use it as a catchall phrase to mean "optimizing your character" when often "minimizing" useless stats never really enters the equation. Grind you way up to the max lever by joining clans, doing activities that give you powerful gear like raids or Trails, and the easiest way is to complete your weekly milestones. Use the higher power lvl weapons or defuse them into another weapon.