A tie is a curved line which connects two notes with the same position on the staff. The tie tells you to sing the first note only, and to hold it for the length of both notes. In the following example, the note G should be held for six beats.
There are two common reasons for using ties. One is that this is the only way of indicating that a note is to be held across a bar line (as shown in the diagram) and the other is that a tie is a way of increasing the length of a note within a bar.
If you look at the written music for the song "Scarborough Fair", you will notice ties used both of these ways. In bar 2 there is an eighth note tied to a quarter note and there are several instances of ties used to indicate notes held across the bar line.
Here is another example to help you become more familiar with the use of ties. Once again, try clapping the rhythms while counting the beats out loud. Then try singing the rhythms while tapping your foot on the beat. Make sure you don’t start tapping your foot on the written rhythms instead of evenly on each beat.