The roll cast is used when obstacles, such as trees, rocks or other anglers, block the angler from making a normal back cast. This situation arises quite frequently (especially on smaller rivers) and the roll cast is a must for all fly anglers to learn.
How to perform a roll cast
Fortunately the roll cast is not as advanced as many new anglers believe. Actually a roll cast is simply an overhand cast with different timing! This video shows the technique for the roll cast in good detail and is very easy to follow. After watching this video I suggest heading down to your local park to practice this technique.