This section introduces a new note-value, the Eighth-Note or Quaver. An eighth-note lasts for half as long as a quarter-note, so in time signatures where a quarter-note lasts for one beat, an eighth-note lasts for half a beat.
Eighth-notes can be written on their own, looking like quarter-notes with curly flags attached to their stems, but for the moment you will see them in pairs, in which case they are joined by a beam. The two bars below show the same music unbeamed, then beamed. They will sound the same.