A tie is a curved line which is used to connect notes of the same pitch. The resulting sound is that of a single tone with the duration value of the sum of the tied notes.
Comparing tied notes to dotted notes can help conceptualize the duration of dotted notes. Play each example while moving the cursor over the notes. Pay attention to the numbers under each note and count out loud as you listen to the example. Match your counting to the speed of the example being played.