What is the meaning of Tick-tock?

The sound of a ticking analog clock.

A warning of some fateful impending event.

To make continual clicking sounds like those of an analog clock; to tick.

When marching, to swing arms and legs on the same side at the same time.

The sound of a ticking clock.

A step-by-step account of an event or timeline.

Source: wiktionary.org