It adds the face fixed-pitch to the inherit attribute of the faces: org-code, org-block, org-table, and org-block-background.

How does one find out that those four faces are the faces to target? This command

M-x describe-face

lists all faces active at point.

I did not check the other three, but when I checked org-code face, its inherit attribute was the symbol shadow and shadow is yet another face name. The above code replaces that attribute with the list:

(fixed-pitch shadow)

so that the combined effect of fixed-pitch and shadow applies to org-code face.

Does this break anything? Yes, I found that when I export as html an org file which contains a source block whose contents is itself an org buffer which contains a source block, Emacs reports error and says that (fixed-pitch shadow) is not a symbol. Probably related to htmlize.