In some of the music you've played, you may have encountered a symbol that says tr. No, this is not an atomic element, more importantly, it means that you should trill that note. A trill consists of two notes that are right by each other, either a whole or half step apart. When you play a trill, you alternate between playing each of the notes for as long as the duration of the note the trill is indicated to be played on. For instance, a trill marking above a half note indicates that you should trill for two beats. One above a quarter note means to trill for one beat and so on. There are certain things to do with trills in different kinds of music, but we'll discuss that more in depth in Musicality and Phrasing.