The viewport is the visible area of a webpage inside the browser window. On a desktop monitor, it's the browser window minus the chrome (address bar, tabs, toolbars). On a mobile phone, it's the visible screen area. When you resize your browser window, you're changing your viewport dimensions.
The viewport matters for CSS in 2 main ways. First, responsive layouts use viewport-relative units like vw (viewport width) and vh (viewport height) to size elements proportionally to the screen. Second, media queries use viewport dimensions as breakpoints: rules inside a @media (max-width: 768px) block only apply when the viewport is 768px wide or narrower.
When a visual problem only appears on mobile or at a specific screen size, the viewport is usually involved. A rule applying only at a certain breakpoint is either showing or hiding something based on viewport width. Loupely Lens captures the viewport dimensions at the moment you click, which is part of the capture context and helps identify responsive layout issues that only manifest at certain sizes.