A fermata (also known as a hold, pause or as a grand pause when placed on a note or a rest) is an element of musical notation indicating that the note should be sustained for longer than its note value would indicate. Exactly how much longer it is held is up to the discretion of the performer or conductor. What it refers to in Baker's novel is a period of time which his protagonist, Arno Strine, can induce during which the world stops, and only he can move around in it freely. He uses this time to undress women and otherwise indulge his sexual desires. The women concerned have no knowledge of what he does and no idea that a period of time has elapsed during which they have been in suspended animation. It isn't as rollickingly funny as it would like to be, but having said that, it isn't as offensive or creepy as you might imagine either. It is, in part, gratuitously pornographic, depicting several scenarios that he either writes about or imagines happening, or that actually do happen (in the novel that is). As those who have read this writer before will expect, he employs his linguistic gifts with great wit at times and to great effect as he writes of, for instance, "fully realised frigments of my invagination".
Obviously, this is not a book for everybody, and if you are at all nervous of sexual frankness, don't start reading - it will offend you. This is a man who is mightily respected for his detailed and dexterously knowledgeable prose. He is, perhaps testing how far he can go without upsetting his literary readership? That, or sheer exhibitionism has got the better of any natural good taste he might have once cultivated. Whether you view this as acceptable in today's climate, or still beyond the pale, will depend entirely on you.