The mainsail leech telltales are ribbons of spinnaker nylon attached at each batten pocket. They stream out beyond the leech and indicate very well the turbulence that the sails are creating, and as already said, turbulence is drag, and drag is not what you want!
The jib telltales are positioned just below each trim stripe, and show you clearly what the air is doing as it meets the sails. If the windward ones are lifting and streaming aft, then you are pointing to high, and you need to come off the wind a little. If the leeward ones are lifting then you are steering to low, and need to helm up a bit. In any event all should be streaming together, and if the upper ones are lifting early, the jib is too twisted, and needs to be sheeted on more.
Many race crews don’t use these, as the telltales will tell them where the sheet lead should be, but for many, a quick and easy indication of whether the sheet car is roughly correct or not is a great help. And, even for race crews, it can help get the initial car position in place quickly after a sail change.