A trill is a rapid succession of hammer-ons and pull-offs, with only the first note being picked. The trill is indicated by the symbol Tr above or below the two notes concerned, with the first note being shown as a grace note. Listen to the following example on the CD to hear the effect of the trill.
Here is a lick which makes use of the trill. Experiment with adding trills to some of your own licks.