These pretty strips of light are plastic diffused backlights, often seen in LCD displays. Normally they're tucked behind the LCD and shine white light from below. Here, we've got an RGB LED variant available individually, and it's really cool! Each LED is encased in a 3.3mm thick strip of acrylic. The acrylic strip is a special type that is very good at diffusing light, so even though there is only a single LED, it provides near-uniform lighting. The other flat side of the strip has a reflector, so you only get light from the top (otherwise, the backlight would lose 1/2 the brightness unnecessarily).