Hmmm Jaime, the telltales are a very simple, effective, and inexpensive way of telling if you are using the wind efficiently. They are approx 1/2 inch wide by 8 inches, or so, strips of fast drying material (you can use string also but strips dry faster). They are generally attached (glued, sewn, etc.) approximately 1/2 to 2/3 of the way up, toward the top of both the main and jib, and they are placed on both sides of each sail. They will also be approx 1/2 to 2/3 of the way back from the front on the jib and about 1/2 way back on the main sail.

How do they work? As your boat moves forward in the wind, naturally the wind passes across your sails, usually on both sides. By looking at the telltales you can see if the wind that is flowing over the sails is moving as quickly and efficiently as it can. The preferred position of the telltales would be pointing straight back, as your boat moves forward. By adjusting the mainsheet and jibsheet (tightening or loosening up the main sail tension or traveler, same with jib) you can adjust how the wind is flowing across your sails. The most efficient use would be to have the telltales facing straight back on both the main and jib and on both sides of the sail. This will usually give you the fastest speed. You look up as you go and adjust as necessary.

If used correctly, you will certainly improve your sailing skills, your speed, and the probability of winning more races against your buddies.

Hope this helps. Sail Safe.
HD.