Original: A hearing dog is a specific type of assistance dog specifically selected and trained to assist people who are deaf or hearing impaired by alerting their handler to important sounds, such as doorbells, smoke alarms, ringing telephones, or alarm clocks.
Simplified: A hearing dog is a special type of assistance dog, who has been selected and trained to help people who are deaf or have hearing problems. The dogs tell their handler when they hear important sounds, such as doorbells, smoke alarms, telephones, or alarm clocks.