존재 is in English language “existence”. It is the state of having being, or simply being. When we say 그것은 존재한다 it means exactly what meaning? There is believed as existence with with essence, essence exemplifies existence, so one knows how essence exists as entity.
The being is the minimal concept which to know what is there, and reality we perceive things. If you say “할머니의 존재는 그자체만으로 나에게 행복이다” (The being of grandma is itself my happiness) says there is dignity and respect from grandma’s existence with you, and it has quality of happiness.
존재론(ontology) is philosophical area concerning 존재. It studies disciplines of nature and types of existence. It forked many concepts and discussions from ancient Greek years, most notable we learn in 101 course of it is distinction between universals and particular, abstraction and concreteness and so on.
The existence is often real objects tangible and visible, however, there are entities that are not so. How can we know about it and what they are? This is main question 존재 is studied by many philosophers.