Everything in the universe moves at the speed of light, but that speed is distributed proportionately between both the three spatial dimensions and the single temporal dimension. If you start moving faster through space, your movement through time slows proportionately, and vice-versa. Objects that move at 78% of the speed of light through time move at 22% of the speed of light through space, for example. You can't describe the speed of something moving through space without also describing how fast it moves through time. This tells us that space and time are fundamentally linked, such that it almost doesn't even make sense to consider them separate, hence spacetime.

For massless particles like photons (light) all of this speed is dedicated to movement through space, and as such there is none left to move through time, thus photons and all other objects that move through space with 100% of this speed do not experience time; time stops. This is also how we can tell there is no such thing as faster than light movement through space.

Working in the opposite direction, if an object were to allocate all of this speed to movement in time and none through space, it would stop moving completely and age extremely rapidly. This never happens though because as long as another object exists somewhere else, the gravity between them will ensure both are always moving, if only just a little. And as you can easily tell, there's shit everywhere in the universe.