The answer is that it means that the word or expression "has unclear boundaries in a given context and purpose".
Vague dialect is extremely normal, particularly in talking. We frequently include words and expressions, for example, about, sort of, kind of, and that sort of thing to make what we say less verifiable and direct. Mostly we use that term when we don't have a clue about the name of something, or to make things which sounds not clear, or to discuss gatherings and classifications.