Sometimes a song can't make up its mind what it is actually about. This song is about both grass and what is known as orthographic neighbors. In particular, it focuses on orthographic neighbors of the word grass which contain the word ass as in donkey. There are quite a few such words which is probably surprising to pretty much no one. I'm sure languages other than English also have lots of examples of this sort of thing, but English is likely to have more examples of entertaining orthographic neighbors than most languages. If you know a polyglot be sure to ask that person whether that last statement is true or not. Otherwise, you'll just have to take my word for it.