What does this nonprinting symbol mean in Microsoft Word?
It's a no width non-break
I've found it here: http://wordfaqs.mvps.org/NonprintChars.htm
Beginning in Word 2000, Microsoft added two new special characters to Word: the no-width optional break and the no-width non break. .... These characters were intended for use with some Asian languages, where characters can be placed beside or on top of one another. Also, the languages may use syntactic constructions that have no spaces between sequential words, unlike the English language which uses spaces to separate words.
from here.