Moisture condensing on the eye lens of your eyepiece or on the front lens of your telescope (in other words, dew) can cut short an evening's observing or ruin a long exposure deep space photograph if it forms during the exposure. If enough moisture condenses, it also has the potential of getting into the optical tube or eyepiece and causing damage.
Dew heaters are low wattage 12 Volt DC electrical heater strips that wrap around the objective lens of the telescope (as well as the finderscope and eyepieces, if desired). These warm the optics above the dew point to prevent the formation of dew. Unlike dew shields, dew heaters don’t merely slow the formation of dew, they prevent its formation entirely.