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.
Here’s a series of 3 trills all in a row.