The phone system that routes calls inside a business: extensions, voicemail, transfers and the main number.
Like the switchboard operator of an old hotel: every call arrives at one place and gets sent to the right room.
A four-person clinic answered one shared cordless phone. Calls during appointments went to a full voicemail box, and patients phoned a competitor. There was no queue, no attendant and no after-hours message.
Business Voice is a cloud VoIP / UCaaS phone system, which means the PBX lives in the cloud. Extensions, attendants, queues and voicemail are configured for you and change when your team does.
Example scenario: A small non-profit had an old PBX in a closet that nobody could program. A board member's extension had rung at an empty desk for a year. Moving to a cloud PBX let the office manager fix it herself in the web console, add an after-hours message, and route donor calls to the development lead's mobile.
Do I still need a PBX if I'm a two-person business?
You need what it does: a main number, a greeting and a way to reach either of you. A cloud PBX gives that with no hardware in the closet.

Every WILDCARD card exists to solve one named problem. Find yours.