A light leak happens when a camera's sensor or film becomes exposed to outside light. Although a light leak doesn't always give you the final result you expected with your photo, it could create a unique, artsy perspective. Usually, a light leak causes discoloration and areas of a photo with high exposure to lighting, but it can yield a one-of-a-kind shot. In older photography, they often led to areas of red glare.
You can follow Photoshop tutorials to learn how to create light leak effects in your photos without actually exposing your camera to light leaks. Light leak effects in Photoshop create light streaks, lens flares, and interesting light patterns. Sometimes, adding a light leak effect can be as simple as adding a premade overlay to your photo.