Sounds have waves. The frequency of sound is how close together the sound waves are. The closer together the waves, the higher the frequency or pitch. You can think of this like the beach during a storm, when waves hit the beach more often.
Dogs and people hear about the same at low frequencies of sound (around 20Hz). This changes at high frequencies of sound, where dogs hear up to 70-100kHz, much better than people at only 20kHz. Dogs hear sound frequencies at least three times as high compared to people.
You may have wondered how those special silent dog whistles work? They make high-frequency sounds that dogs can hear but we can’t. Because dogs can hear higher frequencies than us, there are a lot more sounds for dogs to hear.
They can also hear sounds that are softer or farther away, as far as a kilometre. That means dogs can be more sensitive to loud sounds. This is why some dogs are scared of fireworks or thunderstorms. It is also why a dog might bark at a sound you cannot hear.