HDMI is a digital interface, meaning that it carries digital signals between devices. This is in contrast to older analog interfaces, which carried an electrical signal that was manipulated to match the picture being displayed on a TV.
Digital signals are much better at carrying information than analog signals. This is because digital signals are discrete, meaning that they exist as a series of 1s and 0s. Analog signals, on the other hand, are continuous, meaning that they can take on any value within a certain range. This discrete nature of digital signals allows them to be manipulated and compressed much more efficiently than analog signals. As a result, digital video and audio transmissions can be transmitted over much longer distances without degenerating into noise.