Answer:
It means that he is open and obvious about his emotions, especially if they regard the object of his desire. It can be said of men or women. Such a person is always making an appeal to emotion of some kind. It generally means that the person's emotions are very readable. They are more likely to be hurt by others because they tend to not hide their emotions.
It is an expression for showing that person's feelings. Some people aren't very good at disguising how they really feel, and this phrase was coined for them. They do not hold back their emotions, for good or for bad. It is clear how they feel in each moment, by their body language, facial expressions, stance, movement, words, and tones.