What does the structure "to be+ of+ adj+ noun" mean? As in " This is of cruical importance"

Thanks in advance.

Question

What does the structure "to be+ of+ adj+ noun" mean? As in " This is of cruical importance"

Thanks in advance.

Answer

I would add (because I thought about it more) that the noun in this sentence pattern is not just abstract and uncountable, but something that can usually be measured in some way, and the adjective is something that attempts to define that measurement, or to say that the thing is or isn't measurable. 
Ex.: 
It is of immeasurable importance. (It is very important.)
It is of uncertain depth. (I don't know how deep it is.)
Her face is of incomparable beauty. (I can't compare the beauty of her face with anything else – her face is very beautiful.)
It is of little value. (It isn't worth very much.)

Answer

Literally your example means "The importance of this (thing) is crucial". Or to put it in a simpler way, "This (thing) is extremely important."

And what Charlie wrote is correct: you can only use this structure with certain nouns, for example: "use, importance, consequence, consolation, account, interest, help, significance, value, comfort..." These are all rather abstract, uncountable nouns. And usually the adjective is something that means "a quantity ranging from zero to a lot" or "certain" or "uncertain".

Answer

What does the structure "to be+ of+ adj+ noun" mean?

It doesn't really 'mean' anything. The structure only works with certain collocations. For example you couldn't just put in words, like:

"This is of interesting dog"

"This is of cruCIal importance" --> This is a 'collocation' that happens to use "to be + of etc..."

like:

This is of vital importance.
This is of great significance.
This is of little merit.
This is of certain interest.

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