Musicians often refer to aural knowledge as "ear." In many traditions, musicians "play by ear," learning everything they need to know about the music simply by listening carefully and learning how to reproduce what they hear. "Ear" can also refer to the ability to distinguish whether a performance is correct, for example, whether a note is the correct pitch, played in tune with good tone quality and accurate rhythm and style. "Ear" can also refer to recognizing information about the piece, such as the style, genre, instruments used, chord progression, meter, and key, simply by listening to it. Musicians who have music literacy skills, as well as a highly developed ear, can accurately write down a piece of music they have just heard.
Even if you have had no formal ear training, your ear is almost certainly trained to "understand" familiar music; you have trained it just by listening regularly to your favorite music. If you can tell when a piece is being played out of tune, or with wrong notes and rhythms, that is aural knowledge. It is also "ear" that tells you that you are listening to the beginning of the second verse of a song, or that a recording was interrupted rather than reaching its proper end.