A tie is a curved line that connects two notes with the same position on the staff. A tie tells you to play the first note only, and to hold it for the length of both notes. In the next example the first bar contains four 8th notes and a half note. In the second bar the last 8th note is tied to the following half note.
Here is the same rhythm applied to a riff.
This one contains ties both within a bar and across a bar line.
See how this idea works on the whole 12 bar progression.