Browser
The Quick Version
The program that downloads your website and draws it on screen.
How it actually works
A browser (Chrome, Safari, Firefox, Edge) fetches files over the network, parses HTML and CSS, executes JavaScript, and renders pixels. It also enforces security rules that keep sites from reading each other's data.
Why does it matter?
Your website only exists as pixels because a browser translated your files into them.
Example
Press F12 in a browser to open DevTools and see the errors your site is producing.