Before a user reads a single word, their eye has already mapped the page by weight, size, and contrast. Typography hierarchy is the choreography of that eye movement.
Building a Type Scale
Use a modular scale ù a ratio like 1.25 or 1.618 applied consistently to generate harmonious heading sizes. Tools like typescale.com make this trivial. The hard part is restraint: most interfaces need five levels of hierarchy at most.