What is the meaning of "proofing" in the ...

A proof is the final copy of any photograph or printed page or other reproduction whether text or graphic made for the artist, photographer, author and/or final customer for approval before final reproduction. 

In art, a proof could mean taking an impression of a print while still in the printmaking process to see the situation of the plate while the plate is being worked on by the artist. It could show an incomplete image, though today the term is used to describe a finished work that is identical to the copies going to be printed.

A printer's proof could mean taking a copy while printing in order to check the quality of the print.

hope I helped. Take care. 

A proof in Photgraphy is used when several shots are taken of a subject.  In modeling, they use a "proof-sheet" which shows several small photos of the model on a page. 

Basically in art of any type, is a preliminary version that is checked by the artist or a client before making an investment of time and money to create the actual project.