Essential it's the indispensable, necessary, it's something very important that can't be missed. from latin essential, which refers to the essence, that is, the substance, the main idea, which constitutes the intimate nature of things.
Essential is an adjective of two genders that qualifies something or someone that is of fundamental importance, that has a significant value, that is vital, indispensable.
Many textbooks have the word essential in the title, indicating that they contain the basics, the essentials for learning the subject. Ex.: Essential Mathematics where it presents the subject that teaches the main contents in the area of mathematics, with the purpose of developing logical reasoning and the practice of the subject.
Phrases by important authors containing the essential word:
"The right answer doesn't matter: the key is that the questions are right." (Mário Quintana)
"Each one knows how to love in his own way; the mode doesn't matter; the essential thing is that you know how to love". (Machado de Assis)
"Only seen well with the heart, the essential is invisible to the eyes". (Exupery)