Using the Flash
Your D5500 has a flip-up electronic flash unit built in, but you can also use an external flash (or, “strobe” or “Speedlight”), either mounted on the D5500’s accessory shoe or used off-camera and linked with a cable or triggered by a slave light (which sets off a flash when it senses the firing of another unit’s main flash). Consider using electronic flash:
When you need extra light. Flash provides extra illumination in dark environments where existing light isn’t enough for a good exposure, or is too imbalanced to allow a good exposure even with the camera mounted on a tripod.
When you don’t need a precise preview of ...