"Vanilla JS" is an umbrella term describing web development that remains deliberately lightweight in nature, utilising the browser's native capabilities, without additional overhead of a framework.
Although commonly a reaction to the conceived high churn of JavaScript frameworks, it is useful to understand the growing capabilities of browsers to be able to effectively question whether a particular dependency is actually needed.
Many JavaScript UI frameworks recognise that Web content is componentised and developers want to build re-usable, composeable and modular components. Natively this is now provided through the use of Web Components.
Other capabilities:
DOM querying
DOM events
HTML Data Attributes for storing and accessing data on DOM elements
ES6 String Literals for basic templating