It merely means that its existence is only one factor of a more concrete element of nature.

So an electron is abstract because you cannot wipe out the whole structure of events and yet retain the electron in existence.

In the same way the grin on the cat is abstract; and the molecule is really in the event in the same sense as the grin is really on the cat's face.

Now the more ultimate sciences such as Chemistry or Physics cannot express their ultimate laws in terms of such vague objects as the sun, the earth, Cleopatra's Needle, or a human body.

Such objects more properly belong to Astronomy, to Geology, to Engineering, to Archaeology, or to Biology.
Chemistry and Physics only deal with them as exhibiting statistical complexes of the effects of their more intimate laws.