In the following sentence, how do you explain the usage of "there is" in in terms of meaning and grammar?
"She's teh best there is, as good an actress as I've worked with."
 She's the best (actress) (that) there is/exists.
The underlined portion modifies the implied noun "actress".
 She's the best (that) there is.
The underlined portion modifies substantive "the best".
Meaning: In the field of Acting, there is only one actress who is the best at what she does, and she is that actress. She is the best (actress that) there is (in her field of work).
Hope that helps.