Extra-linguistic universe means the subjectively understood world of each individual, which provides him with things to talk about. It can consist of his actual or imagined personal experiences, observed or reported experiences of others, visions, aspirations and so on. A lot of what is needed to understand these is shared by all mankind, which is what makes communication possible across individual and group boundaries.
If a person, for example, says to a rock, “Move out of the way” he might not have performed an illocutionary act to the satisfaction of the philosopher. But that is beside the point as far as the linguist is concerned. For linguist it is as good a command sentence (expressed by the imperative mood in this particular case) as any, whether it is addressed to a man, a dog or a rock.