Hard of Hearing (HoH) refers to someone who doesn't hear well. This may be because they were born with a hearing loss or they may have lost some or all of their hearing later in life.
Many hard of hearing people don't know that they have a hearing loss. Some simply deny it, even though they may know that their hearing is diminished. Some people who are completely deaf may consider themselves hard of hearing. In all, nearly 10% of all people have some level of hearing loss.
It's estimated that 28 million people in the US have some level of hearing loss. The largest part of that group (perhaps 25 million or more) can hear well enough that with proper hearing aids or assistive listening devices, they can continue with their spoken language as their primary communication mode.
Usually hard of hearing people continue to rely on their spoken (or written) language as their primary mode of communication, though some hard of hearing people do use sign language or Cued Speech as a supplementary support.
While many older people do experience some loss of hearing, hearing loss is not a problem for just older people ... many babies are born with hearing loss and many children and young adults have hearing problems.
Until recently, almost nothing could be done to help someone who had a hearing loss, but now there are many things that can help (see History of Hearing Loss).
Self Help for Hard of Hearing People, Inc. (SHHH) is the most significant organization for people interested in hearing loss. If you have a hearing loss or care about improving things for people with a hearing loss, then you should join SHHH.