I have tooth or gum sensitivity on a tooth but the dentist said it's not a cavity. what does this mean?

Tooth gum sensitive: There is no way to know without a clinical and visual exam. What was your dentist's diagnosis? If your dentist does not know what is causing the sensitivity, consider seeking a second opinion.

It's not a cavity...: ... according to your dentist. It may be a loose filling, gum recession, a tooth fracture, bruxism, or periodontal issues. Very hard to diagnose these issues over the internet, sight unseen.