Basically, Magento widget is a Magento extension with definite functionality in your store. The Magento widgets assist in better user experience and functionality of the store. Customize a widget to extend the functionality of the core widgets or create a custom widget for Magento 2.
Widgets are useful in the administration to add interactive interfaces and features in the front-end of Magento 2 store.
For more info: https://meetanshi.com/blog/create-a-custom-widget-in-magento-2/