The trail off is actually another type of slide where instead of sliding to a note you start by playing a specific note and then slide off the note. As you slide away from the note, gradually release pressure off the string and allow the sound to fade. A trail off is indicated by a wavy line moving diagonally down or up from the note.
Here is a lick which uses trail offs.
These next few licks also use trail offs, combined with other techniques already learned.