Incapable means unable to do something, lacking the power, skill, or capacity needed for a particular task. When something is beyond your current abilities, you're incapable of doing it. A kindergartner is incapable of solving algebra problems. A bicycle is incapable of flying.

The word often appears with “of”: someone might be incapable of lifting a heavy box, incapable of understanding a complicated concept, or incapable of keeping a secret. Notice that being incapable of something isn't necessarily permanent. You might be incapable of playing Beethoven's Fifth Symphony on piano today, but with years of practice, you could develop that capability.

Sometimes people use “incapable” more absolutely, to suggest a fundamental inability. A rock is incapable of feeling emotions. A fish is incapable of breathing air. In these cases, the limitation is built into the very nature of the thing.

The word can sound harsh when applied to people, so use it carefully. Saying your friend is “incapable of telling the truth” is a serious accusation. But describing yourself as temporarily incapable of something you're still learning shows honest self-awareness, like admitting you're currently incapable of running a marathon without training yet.