I'm inclined to agree with Jibudo that what you're looking at, up above the staff, are the chord notations. If they look something like E7 or E-7 (major and minor dom. seventh chords) or sometimes simply E (a triad chord based on E as the root) or perhaps something like C/G (a "slash" or inverted chord where the fifth interval to be played is the G below C and not the G above C), then you're looking at chord notations. They won't always be the same as the note you're given to play in a written bass part on those rare occasions when you're given a bass part.
Though notionally written for chord instrument players--mainly keys and guitars--chord notations are vital to many (most?) bass players in formulating their own bass lines as they indicate the appropriate root or inversion to play as well as suggesting chord tones from which one can build a nice sounding bass melody that will work harmonically with the main melody.
That's maybe more than you wanted to hear right now, and others can no doubt explain it better.
Jeff
