'User Interface (UI)' in web design and development is about the interactive space where humans and machines communicate. The objective of effective UI design is to streamline the user's interaction to be straightforward, intuitive, and efficient, with an emphasis on both aesthetics and responsive interactions.

A proficiently designed UI ensures effortless access to a website's or application's features. It comprises components like buttons, menus, and forms, along with visual elements such as color schemes, typography, and layout. Optimal UI design is not solely about looks; it's equally focused on enhancing usability and enriching the user experience.

Creating a UI entails understanding user needs and behaviors, crafting the interface with a user-centric approach, and incorporating interactive elements and technologies for seamless navigation and functionality. The UI should be accessible, accommodating users with various abilities and preferences.

As technology advances and user expectations shift, UI design continuously adapts, embracing contemporary trends and principles like material design, minimalism, and emotional design. Integrating UI design with user experience (UX) is crucial for developing digital products that are visually compelling and functionally efficient.

To conclude, the UI plays a pivotal role in web and app design, significantly impacting usability, user satisfaction, and the effectiveness of human-computer interactions. A well-constructed UI is the bridge between the digital and real worlds, rendering technology both accessible and enjoyable.