An image is backlit when the light source is on the far side of the subject in relation to the camera. It means that there’s more light coming from behind the subject than is directly on the subject itself. It’s often used to separate the subject from the background to make a subject more dramatic, or to make a silhouette. In photography, a backlight (often the sun) that is about sixteen times more intense than the key light produces a silhouette. A fill flash used with a backlit subject yields more even lighting.
The vertical angle of the backlight can change the effect. A low angle can make the light hit the camera lens, causing a lens flare. A high angle can make the subject’s nose extend out from the mostly vertical shadow of the head, producing a potentially unwanted highlight in the middle of the face.
Sources: Pixabay, Wikipedia, Susan Wingfield Lamar High School