A tie is a method of increasing the time value of a note. It is indicated by a curved line connecting two consecutive notes of the same pitch. Ties are commonly used to create syncopated rhythms, as demonstrated in the following example.
In the first bar all the written notes are played, while in the second bar the fourth 8th note is tied to the following note. The difference should be easy to hear.
Here is a rigg using the thythm from bar two of the previous example.