In traditional notation, a tie is a curved line that connects two notes with the same position on the staff (and therefore of the same pitch). A tie indicates that you play the first note only, and hold it for the length of both notes. A tie is not necessary in Tab notation where you can simply follow the count. The use of ties is another common way of creating syncopated rhythms.
This example demonstrates the use of ties both within a bar and across bar lines. A tie is the only way of indicating that a note is to be held across a bar line. Do not confuse the tie with the slur, e.g. hammer-on, pull-off, bend or release, which connects two notes of different positions (pitches) on the staff.