Have you ever sung in public, only to have your family ask you to never do that again? Or, have you played the piano and your sour notes had people scrambling for an exit?
If so, you may be tone-deaf. Don't worry too much if you are. It's not a problem with your hearing; it's just that your brain isn't processing clear differences between the highness and lowness of sounds, which most people can hear.
Pitches are the distinct sounds that make it easy for most people to tell the difference between a good singer and a bad singer. Pitches also make the sound of specific notes, like on a piano keyboard, distinct from one another. So, when you hit a C-sharp instead of an E-flat, you know it.
Unless you're one of the estimated 2% to 5% of people who can't tell the difference. They're tone-deaf. If you think you might be, or if you want to prove to your family once and for all that you have a beautiful singing voice, the National Institute on Deafness and Other Communication Disorders has a fun test to determine if you're tone-deaf or not.