In modern physics, spacetime is considered to be a fundamental concept that combines three dimensions of space with one dimension of time into a four-dimensional framework. According to the theory of general relativity, massive objects deform or curve spacetime, and this curvature influences the motion of other objects in their vicinity.
When we say that spacetime is not a "real thing," we mean that it is not a substance or entity in itself. Instead, it is the underlying structure in which all physical phenomena occur. It is more accurately described as a mathematical abstraction that provides a framework for understanding the behavior of matter and energy.
The gravitational field of a massive object is not directly communicated or mediated by something tangible between objects. Instead, the presence of a massive object causes a curvature or deformation of the surrounding spacetime. This curvature influences the motion of other objects by determining the paths they follow through spacetime.
In a sense, you can think of the curvature of spacetime as a "background" that tells objects how to move. An object in the vicinity of a massive object, such as a planet orbiting a star, follows a trajectory that is influenced by the curvature of spacetime caused by the star's mass. This trajectory appears as the motion we observe as the gravitational force.
To summarize, the gravitational influence of a massive object is not directly mediated by anything between objects. Instead, it is the curvature of spacetime caused by the presence of mass that informs the motion of other objects, leading to the gravitational force we observe.