BSI is Back Side Illumination in a digital camera sensor.
It gets very technical very quickly, but the gist of it is that BSI is a new way of organising the layers in the digital sensor of camera. The new layering allows more light to travel to the pixels that read the light.
The big benefit of BSI is more precise reading of the light. Some say that the signal to noise ratio is improved, others that the dynamic rang is increased. If you compare the spec sheet of a traditional sensor with a back side illuminated sensor, the difference should appear as better specs for the BSI sensor.
In my experience, the improvements are shallow. I have both the Nikon D750 and the Nikon Z6ii, i.e. one camera with a traditional sensor and another with BSI. For my style of shooting (outdoor photography), I can hardly tell the difference, and I think it is because we are comparing some very good sensors to some very, very good ones!
Related reading
What is a camera image sensor?
