A trill is performed by quickly hammering on and pulling
off two notes. It's usually represented by a "tr"
and a squiggle line like seen for a vibrato. In the example
below you can see a trill in tablature. Place your 1st
finger on the note on the 7th fret and use your 3rd finger
to rapidly hammer-on and pull-off on the 9th fret-which
is in parenthesis.