In traditional notation, a tie is a curved line that connects two notes with the same position on the staff. A tie indicates that you play the first note only, and to hold it for the length of both notes.
In the 1st example here, pick the D note and hold it for 3 counts
In the second example, pick the D note and hold it for 1p counts
A tie is not necessary in tab notation where you can just follow the count.
A tie is necessary if a note is played over a bar line, as in the following two bar riff. When ties are used to connect quarter notes and eighth notes, as in the examples given, an "off the beat" feel is created. This is called syncopation.
The following Blues in E uses a tie within the bar, in bars 2, 4, 6, 8, 10 and 12. Practice this timing separately before playing the entire Blues.