The sentence refers to the level of difficulty in interpretation of spoken language. The word "boundary" means "a line which marks the limits of an area; a dividing line:" This means literally "地域边界线” a line on the border of a piece of land. Word boundaries are the beginning and the ending of a word. In writing, word boundaries are conventionally represented by spaces between words. In speech, word boundaries are determined in various ways. Ref.(http://grammar.about.com/od/tz/g/Word-Boundary.htm)

Here it means limitations. Limitations of the words we are using