Every game is a loop: read input, update state, render frame, repeat. But the naive approach — update and render once per iteration — ties game speed to frame rate. On a fast machine the game runs too fast; on a slow one it crawls. The fixed timestep pattern solves this elegantly.

Fixed Update, Variable Render

Accumulate real elapsed time. Run physics/logic updates in fixed-size steps (e.g. 1/60th of a second) until the accumulator is drained. Render as fast as possible, interpolating between the last two physics states for smooth visuals. Physics is deterministic and frame-rate-independent. Rendering is as smooth as the hardware allows.