This is quite the recondite passage, and it requires reference to paragraph 70 to fully interpret. Briefly, in paragraph 70 Berkeley considers a critique that might be brought upon the general argument of his treatise—that there is no unperceivable substance. The critique (in paragraph 70) goes as follows: there may be particular parcels of matter, of which we have no immediate knowledge or sense of, but of which God himself is aware and which he uses to permit us to form images in our minds based on our sensation of external stimuli. Therefore, a kind of substance, not perceivable to humans but perceivable to God himself, does exist somewhere outside of man’s immediate perceptions.
Berkeley responds to this in paragraph 71, basically by saying that this indeed could be the case but that it still does not refute his underlying argument. When he says that the question being considered is no longer “of a thing distinct from Spirit and Idea,” he means to say that the perceiver and the perceived thing are no longer separate entities. This is because God, as the perceiver, is total and lacks nothing in his own existence. In any case, Berkeley concedes that this indeed may be the case—the argument that the perceived thing and perceiver may be the same object (God)—thus proving that matter can exist outside of human perception. However, he contends that the argument is so nuanced and sophisticated that it doesn’t really detract from the general points he had been making in his work up to that point.
Berkeley illustrates this idea with an allusion to a musician. A musician is driven by the musical notes on his paper to create music, which the spectator in the audience consequently hears (i.e., he perceives it). However, the notes themselves are never perceived by the audience. Rather, they are used by the musician to allow the audience members to experience a particular kind of perception (the sound of the music being played). This is, metaphorically, the same way God can use unperceivable matter or ideas to stimulate certain perception in the human mind, even if humans themselves never directly encounter the objects that have led to the perception in the first place (which only God knows).
The critical thing to keep in mind here is Berkeley’s last sentence:
Besides, it is in effect no Objection against what we have advanced, to wit, that there is no senseless, unperceived Substance.
Here he was no doubt taking influence from the work of Spinoza’s Ethics, whereby he (Spinoza) defines God as the primary substance and cause of all auxiliary substances. This is an incredibly convoluted ontological philosophy, but all you really need to know is that Berkeley believed that nothing could be known outside of sense perception; all the matter of the world only existed as images that resided in the mental world of our perceptions (and not outside of it, in the “real” world). This is what he means in this last sentence. He further borrowed from Spinoza’s definition of substance—if the primary substance is God, from which all other matter originates, and if God himself is unknowable and unperceivable, then true substance (or matter) is also not perceivable. All humans have access to are the images they form in their head, not the real substratum of matter itself. This is because, like the musician and his notes, God only gives humans the perception of a thing (the music) and not the thing itself (the musical notes on a score).