A worldline is the complete path an object traces through spacetime over the course of its entire history, from one moment to the next, strung together into a single continuous thread. A person sitting still has a worldline too, since even without moving through space they are still moving forward through time.
Every event a particle experiences, lighting a candle, kicking a ball, launching into orbit, sits somewhere on its worldline. The worldline is what turns a scattered collection of separate moments into the connected story of one object’s existence.
In general relativity, an object moving under gravity alone traces a very specific kind of worldline called a geodesic, the straightest path available through curved spacetime.