Meaning of "it isn't all it's cracked up to be"

[Anchela]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #65 "What a Cracker", question 8

Well, I must admit that I was a bit disappointed with the exhibition after all the praise it's received -- it isn't all it's cracked ......... to be.

(a) off
(b) up
(c) over
(d) in

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #65 "What a Cracker", answer 8

Well, I must admit that I was a bit disappointed with the exhibition after all the praise it's received -- it isn't all it's cracked up to be.

Correct answer: (b) up

Your answer was: incorrect
Well, I must admit that I was a bit disappointed with the exhibition after all the praise it's received -- it isn't all it's cracked off to be.
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I don't understand the expression: "it isn't all it's cracked up to be." What does it mean?

Anchela

[Conchita]

"It isn't all it's cracked up to be" is said when we are disappointed because something is not as good as expected or as we were told.