HACS (Home Assistant Community Store) is a custom integration that provides a UI to manage custom elements in Home Assistant.

Basically HACS is where most of the new integrations are accessible. Home Assistant itself includes a lot of intergations, and many more can be added via the plug-in engine, but HACS adds a very wide range of widgets and integrations which are maybe more experimental, but that increases the types of devices you can integrate.

Installing HACS is a bit more complex than installing a classican add-on. You should carefully read this page to understand what you need to do.

First of all, check the requirements.

Download HACS, following the instructions here. Use the OS/Supervised instructions if you have installed Home Assistant on bare metal as i suggested.

Last step is to to complete the basic installation, according to this page.

After HACS is installed and started, you will find a dedicated HACS page in the Home Assistant sidebar. From this page, you can search for and install the various additional stuff provided by HACS.