Is there a word for "a word or phrase whose lack of understanding by a reader does not stop his understanding the gist of the sentence it's used in"?

Solution 1:

You've cast the example as a noun, but there are some adjectives that do the job nicely. Some can function as noun or adjective:

• peripheral (n or adj)
• incidental (n or adj
• irrelevance (n)
• ancillary (adj)
• inessential (adj)
• inconsequential (adj)
• secondary (adj)
• expendable (adj)
• distraction (n)

A few words that are not what you're looking for, but are close:

• placeholder (Implies that the word has no intrinsic significance)
• fluff (negative connotation implying that the word is better omitted)
• MacGuffin (staying with the cinematic theme, a thing that is given more significance by the context than by its intrinsic qualities)