The subwoofer is a specialized speaker that is designed to reproduce extremely low frequencies.
You then connect the speaker outputs on the subwoofer amplifier to the speaker terminals on the passive subwoofer.
In addition, some inexpensive home-theater-in-a-box systems incorporate a passive subwoofer, such as theOnkyo HT-S7800.

Images courtesy of Amazon
This throw in of subwoofer is self-contained.
Which Is Better Passive or Powered?
Whether a subwoofer is passive or powered isn’t the determining factor on how good the subwoofer is.

Amazon
This makes them very easy to use withhome theater receivers.
In most cases, it is more cost-effective to buy a powered subwoofer in place of a Passive Subwoofer.
The Exception to the Subwoofer Connection Rules
Many powered subwoofers have both line input and speaker connections.

Klipsch
This can be implemented in two ways.
If so, then you should use a powered subwoofer.
The giveaway is that it does not have a subwoofer line input and only has speaker connections.

Onkyo USA
If so, you will need to purchase either an additional amplifier to power the subwoofer.

Jamo

Robert Silva