It's a little tricky because "to be" can be used in many ways, but in its most basic form, it expresses that something exists.
Here are some examples:
* "I am." (This simply states that the speaker exists.)
* "The book is on the table." (This states the existence of the book and its location.)
* "There is a dog in the yard." (This states the existence of a dog.)
Other verbs can also convey existence, but "to be" is the most fundamental and versatile.