Typically, what it would mean is that in this diagram till about here the waves that you see will be due to forward wave, but the wave that you will see here will be addition of this and this, you can see as the diagram shows the trough will increase- can you see that?- and if you come here again, the two waves will add, but when you come here, there will be three waves that are being added up, and when you come here there are four waves which are being added. So, the resultant wave may look something quite different from any of these, but one thing to notice is the forward wave crest, which is a most prominent of all this, stay as it is, that is not disturbed by the other waves- do you understand this?