It means that people always like to talk about something (将来future、明天tomorrow). but listener should judge the speaker's emotion accoding to the speaker's mood and the context. in most cases, the sentence conveys the speaker dislike. for example: A said to B ：他总喜欢把将来和明天挂在嘴边。 A maybe consider C is a little unrealistics ,not dwon-to-earth and aim too high.

“挂在嘴边" to keep saying something over and over. He keeps saying 明天 and 将来 over and over。