This is a core block and thus available by default in Gutenberg WordPress editor. No download required.
WordPress Cover Block in Gutenberg allows you displaying a short Text with a background Image over it. Using this block you can create some attractive post or page headers as well.
Cover Block Features
Cover Block has alignment options in the toolbar. It also has the option to write text inside and select a background image or video. From the block settings panel in the sidebar, you can select background overlay color and adjust its opacity. You may also set it to be a fixed background image thus it behaves like a parallax section.
This is how cover block looks on the frontend.
Cover block added to a page.
Cover block sidebar options.
Cover block overlay options.