A spaceship repeats in a new location
A spaceship is a finite pattern that returns to its shape after a fixed period, but translated on the grid. The glider does this diagonally every four generations. The lightweight spaceship moves orthogonally with period 4.
This is the practical difference between an oscillator and a ship. Both repeat. Only the ship changes position.
Speed notation is a compact measurement
Life uses c as the maximum speed at which information can travel across a range-1 rule: one cell per generation. A glider travels at c/4 diagonally. Classic lightweight, middleweight, and heavyweight spaceships travel at c/2 orthogonally.
Those fractions are not decorative notation. They tell you how many ticks a signal needs before it can reach another object.
The glider became the default signal
The glider is small, frequent, and easy to recognize after four ticks. That made it the standard moving object for later Life engineering: streams, reflectors, guns, syntheses, and logic constructions all rely on controlled moving signals.
For a new learner, the simplest experiment is still the best one. Load a glider, step four times, and compare the translated shape with the starting phase.
Working takeaway
If a pattern seems to move, pause after its period and compare its bounding box. A real spaceship repeats its phase after translation.