would like to know the meaning of the next sentence.
It's always shocking to see one's existence reduced to a blurb.
From the Google dictionary, the definition of a blurb is : A short description of a book, movie, or other product written for promotional purposes and appearing on the cover of a book or in an advertisement.
This is like the movie advertisements that quote reviews saying "Wonderful!", "Fascinating!" or "Best Movie of the Year!".
Calvin is saying he is shocked to find that his entire existence as a person can be reduced to one single word - and not that good of a review!
*** Not a teacher ***