Viewport
The viewport is the visible area of a web page within a user's browser window or device screen. On a desktop, the viewport is roughly the full browser window. On a smartphone, it is the small screen of the device. Responsive design ensures your website looks and functions correctly within any viewport size.
Real-world example
The viewport is the frame through which you look at a painting. A large frame (desktop) reveals the entire composition at once. A small frame (smartphone) shows only a portion at a time — the painting must be designed to work in both cases.
Why it matters for you
The tag — a single line of HTML — is essential for correct mobile rendering. Without it, your site displays at desktop width on mobile, making it tiny and unusable. Every professionally built site includes this tag.