Subwoofers are offered in various sizes and power outputs for different sized rooms. They are also offered in both passive or active (powered) formats for in-room or hidden applications.
Powered subwoofers add impactful bass to music and movie soundtracks. They connect to your receiver or amplifier via a subwoofer cable (or wirelessly using a USB power supply on many models) and connect to power for their internal amplifier.
Subwoofers can come ported or sealed and/or include passive radiators, all of these considerations depend on the listening room and material being played and the desire for the subwoofer cabinet to be discrete or not.
Speaking of discrete, passive subwoofers are also an option and while they require an external amplifer, they can be installed in-wall or in-ceiling to be completely hidden from view.
Finally, if you are a keen to squeeze every deep note out across your entire listening area, multiple subwoofers are better than one.