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.
A worldline is specific to a single point-like object, but the concept naturally extends further. In string theory, an object with actual length, like a fundamental string rather than a point particle, traces out a two dimensional surface through spacetime instead of a one dimensional line, called a worldsheet. Light itself has a special kind of worldline called a null worldline, tracing out exactly the boundary of a light cone rather than sitting inside it, since nothing with mass can ever quite catch up to the path light takes. Comparing worldlines is also how physicists formally describe the twin paradox, two people can start and end at the same two spacetime events yet trace completely different worldlines between them, accumulating different amounts of proper time along the way.
On a standard spacetime diagram, with time running up the page and space running sideways, an object at rest traces a perfectly vertical worldline, while a moving object’s worldline tilts, and the faster it moves, the closer that tilt approaches the 45 degree line that marks the path of light itself, a boundary no worldline belonging to an object with mass can ever quite reach.