A straight 8 in music refers to a subdivision of notes that parts from the quarter note. When divided, quarter notes will become eighth notes. The role of eighth notes in music is to divide musical beats evenly in half. The reason why they are referred to as "straight" is because the division of the beat is clean and even. Most music is asked to be played in a straight 8 style.
In classical music, eighth notes are exactly one half as long as quarter notes. However, the prefer usage for eighth notes is for Jazz music, whose one-and-two rythm creates the unique atmosphere for which traditional Jazz is known today. Musicians refer to specific straight 8 notes as "shuffle" or "swing" eighth notes depending on the style that they play.