Cats are not as social as many other species and they do not communicate in the same ways we do. Contrary to what people believe, cats primarily rely on smell, not sight, to communicate with other cats, locate food, and detect predators.
Cats communicate by "marking" objects and other animals, these marks are scents (called pheromones) released from special glands located in their forehead, cheeks, tail base, and paws when they scratch and rub their body on objects.
Cats also do not rely too much on sounds from other cats, so they do not pay particular attention to the verbal sounds we humans make.
Allowing your cats to mark scratching posts and other appropriate objects in the home will reduce the temptation for them to mark using urine. It is important that indoor cats have enough litter in a clean litter box to hide urine scents after covering. In the wild, cats urinate in new clean spots all the time; if we want them to use the same spot each time, we have to keep it clean!