VoIP stands for "Voice Over Internet Protocol". It's an alternative to wired phone. Wired phones transmit sounds over telephone wires. VoIP technology uses a special device that plugs into a high speed Internet connection (if you have one) and uses computer technology to transmit sounds digitally through the Internet.
VOIP contracts supply you with that special device and you are normally billed a flat rate for unlimited phone calls.
There are some limitations, such as the way 911 (emergency) calls work (or not) and the potential of losing phone service if your high speed Internet connection is down or if the power is out.
VOIP does usually provide caller-id as a part of the package, so that could be an advantage for people with hearing loss.