With its wide range of device profiles, the Minecraft UI is a particularly complex beast. Every on-screen view and control must support pretty much every reasonably modern screen size, from tiny mobile device screens to full, hi-rez, computer and console displays. Additionally, they must all support a wide variety of interface types, including: mouse & keyboard, touch screens, gamepads, and VR point and shoot.
In order to help with all of this, and do so in a way that fully supports the signature, clean and crisp pixel-art style of the Minecraft UI, the game engine provides breakpoints for screen sizes which switch between GUI scales. Each of these scales multiplies every pixel rendered by an appropriate scale factor. Thus, at GUI-3 every pixel (or texel) is rendered as a 3 pixel by 3 pixel group. Of course, this is only one part of a more complex rendering system that includes other elements like aspect scale definitions and a user's ability to manually change their preferred GUI-scaling.
Ok, that was pretty dry... Suffice it to say that there are lots of considerations that have to be kept in mind while designing for Minecraft.