What is inside a sound bar?

A marketing device, for people that don't care about sound, but need to be sold something that's about sound.

The device uses several cone speakers in a line with a processing device to create the impression of surround sound from a flat bar instead of speakers around the room. The 'duct tape' version of a 5.1 surround system.
So it is a cheap substitute for a true 5.1 system.
When you want something more than just the TV speakers, but can't afford a real receiver with the required speakers.
Now some of these things cost two grand. they are still a slapped together cheap substitute for a real surround system.

While it represents a compromise from a dedicated multi-speaker surround setup, high quality sound is obtainable from soundbar type products, particularly if you use them with a quality subwoofer. Look for the higher end Yamaha or Definitive Tech models.

It is possible to get better sound for equivalent or less money than a soundbar, but they are an elegant solution for people who don't want electronic clutter in their living spaces.