Spacetime is the framework Einstein introduced to merge space and time into one single structure, rather than treating them as separate stages on which physics plays out. It has three dimensions of space and one of time, woven together so tightly that different observers can disagree about how much time passes or how far apart two events are, yet still agree on the combined spacetime interval between them.
In general relativity, spacetime is not a rigid, fixed backdrop. It is flexible, and mass and energy actually bend and warp it. That curvature is what we experience as gravity, an object does not get pulled toward the Earth by some invisible force, it simply follows the straightest possible path through spacetime that has been curved by the Earth’s mass.
This reframing is why light bends around massive objects, why clocks run slower near a black hole, and why GPS satellites need to correct for the different way time flows at their altitude compared to the surface of the Earth.