The terms Backside Illuminated (BSI) and Frontside Illuminated (FSI) refer to the light guide in digital cameras and in smartphone cameras. In the first case, it is backside illuminated, in the second case, it is frontside illuminated
It is about the direction from which the light hits the image sensor. In the case of Backside Illuminated, the image sensor with its light-sensitive structures is located directly behind the microlens and the color filters. This results in more light hitting the light-sensitive photodiodes, giving them better pixel performance, especially in low-light conditions.
It's a different story with frontside illuminated. With this frequently used technique, the light path to the light-sensitive photodiodes may be restricted or there may be interference between the conduction structure, which is located in the upper area of the image sensor, and the microlens.