Primary camera in mobile is referred to as the rear camera.
Use of primary camera
Still image capturing
Motion image capturing
The front camera is referred as the secondary camera. This is commonly known as selfie camera.
Use of Selfie camera
Image capturing, mainly of own face.
Though the dual camera means presence of two cameras. But in this mobile era the dual camera is actually mean either two dual rear cameras or dual selfie cameras. In the dual camera set up the first camera is used to take the image as of the other camera are doing and the other camera is used for depth of field.
Advantages of dual lens camera system in smartphones
Better depth of field in images
Better picture clarity
Faster focus can be achieved