Suppose it's a stray cat; back off slowly. If it's a pet cat, try to walk away and examine the causes.  If your cat seems hungry or thirsty, immediately provide some nourishment. If your cat is annoyed by any background sounds, tune it off. If hissing is due to signs of pain or illness, take the cat to the vet or slowly attempt to provide care through the use of food, water, or toys.  If hissing is because of exhaustion or fear, leave the act as it is, and it'll be fine!