The simple formula for propagation delay is, propagation delay = distance traveled / propagation speed. This digital circuits formula calculates the time needed for the propagation of a signal change from one node to the next through communication media.
Propagation delay is determined in an effort to avoid glitches in digital circuits. Propagation delays are short in simple circuits, but as the circuit becomes more complex, delays become extended. If a signal change is not registered soon enough by a specific gate, the change is not properly propagated and errors occur. Although the signal change goes through eventually, the initial glitch has already created a domino effect with subsequent gates.