KTP lasers utilize laser energy at 532nm, which is coherent green light. (KTP stands for Postassium--chemical symbol K, T stands for titanyl, and P for phosphate: the potassium titanyl phosphate crystal is used to double the frequency (halving the wavelength) of a Nd-YAG 1064nm near-infrared laser).
At this green wavelength of 532nm, there is pretty good absorption of the laser energy by two things present in skin, hemoglobin and melanin, the skin suntan pigment. The rest of the skin doesn't absorb this wavelength of laser energy well, so when you have either tiny superficial blood vessels you want to eliminate, or brown spots (sun spots, freckles, keratoses--not moles or skin cancers) that you want removed or lightened, this is an excellent choice.
Your doctor is giving you good advice. Listen to him. Online research should supplement, not replace, consultation with experienced, board-certified physicians that you trust. If your doctor won't take the time to answer good questions, find another who will.