what does it mean when a person can't hear at a high frequency?

Answer 1

Most likely that the person has damaged their inner ear to the point where high pitched frequencies are not recognized.

The 'hammer' in your inner ear is most likely the problem of this.
Unfortunately, there's not yet a way to fix it, besides a hearing aid:D

Answer 2

As you get older, the range of frequencies you can hear reduces. Adults usually can't hear high frequency, as teenagers and younger people can.