What are Material UI Icons?

The @material-ui/icons package, which is not included if you only install the @material-ui/core package, is a package for rendering Material icons. This package can be installed independently of Material UI if you're just looking to render Material icons.

As you can read on the Material UI icons documentation:

These components use the Material-UI SvgIcon component to render the SVG path for each icon

Where can I see the full list of Material UI Icons?

You can search the full list of icons here. Just use the name of the icon when importing as above, for example import AlarmIcon from '@material-ui/icons/Alarm'. You can name the component whatever you want since its the default component exported from that directory.