Just Search is an Assist replacement (the app that launches after long pressing home) and cannot be launched via the launcher. To use it, navigate to the device settings and change the default Assist app.
Just Search provides a simple way to search from everywhere without needing to launch the heavy (in terms of data and resources' needs) Google search app. Just Search uses DuckDuckGo by default, but allows opting for a multitude of predefined search providers instead. It also allows setting a custom search URL so that any search engine is supported.