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. A tie is not necessary in Tab notation where you can just follow the count.
The dotted half note B is another option of writing the same rhythm A with a tie.
The following examples all incorporate the low F note and the tie. A tie is necessary if a note is to be held over a bar line, as in the following bass lines, which also contain ties within the bar.