Leonardo UI was introduced with Qlik Sense 3.0. Here's a description from the documentation -
Leonardo UI is a UI component framework, designed to be used as a complement to other frameworks. It is non-obtrusive in the sense that it contains no global styling but will use your page defaults. You can also use it standalone with a CSS reset to get consistent styling of your components.
How can I use Leonardo UI?
Leonardo UI is available to use in Qlik Sense mashups, extensions, and widgets as of Qlik Sense 3.0, and you do not have to do anything extra to begin using it in those contexts.