A disconnect is a device that shuts off all flowing power safely in the event of a service call or emergency.
You will find two in a residential PV system. One, located between the panel DC wiring and the inverter. The other, between the main service panel and the inverter.
Sometimes, inverters have built-in disconnects.
Residential power systems rely on the smaller-sized disconnects, which are rated by the amperage they are allowed to handle. They come in 30 amp and 60 amp versions.