When someone says they can "hear" music, they are typically referring to an internal auditory experience in their mind. It's a phenomenon known as auditory imagination or auditory imagery. In other words, they can mentally recreate and perceive music in their thoughts without any external auditory input.
For individuals who have a strong ability to "hear" music in their mind, it can be similar to the experience of actually listening to music with their ears. They may be able to imagine specific melodies, harmonies, rhythms, and even lyrics in vivid detail. Some people can also mentally hear different instrumental parts or imagine the timbre and tonal qualities of various instruments.
However, it's important to note that this internal auditory experience is subjective and can vary from person to person. Not everyone has the same level of auditory imagery ability, and some individuals may have a more limited capacity to recreate music in their minds. Additionally, the intensity and clarity of the imagined music can also differ among individuals.
Overall, when someone says they can "hear" music, it means they have the ability to mentally perceive and imagine music as if they were actually listening to it, but the experience is generated internally rather than through external auditory stimuli.