It's often referred to as a turbo compound setup, altough that term is used for many different setups including two turbine stages.
In short:
Exhaust gases drive the turbine of the smaller turbo. After that the exhaust gases drive the turbine of the larger turbo. The smaller one need to have a large wastegate.
The cold air intake goes first to the compressor of the larger turbo and then through the compressor of the smaller turbo. The to the engine via an intercooler.

Now, at low revs, the engine practically works as it only had the small turbo. When the revs rise, the wastegate of the small turbo will be wide open, and the larger turbo is spinned up. The boost is now built by the two compressor stages makin gthe system go further up and also more efficiently.