The phrase "key essentials" refers to the most important things or elements that are necessary for something to work or be successful. It essentially means the essential or crucial parts that cannot be overlooked. Full definition
Key essentials for your purse A bedtime routine proven to help my baby sleep? (babycenter.com)
I eventually learned that there were just a few key essentials I need to have in order to create a memorable and festive environment for me and my guests. (dukemanorfarm.com)
In guinea pigs and primates, including man, one key essential nutrient is vitamin C. (cedarbayouanimalclinic.com)