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.