What is gravity, really?
One-line takeaway
BLUF: In modern physics, gravity is not a "force" in the usual sense but the effect of mass–energy curving spacetime.
A fundamental explanation of what is gravity, really?
The Explanation
In modern physics, gravity is not a "force" in the usual sense but the effect of mass–energy curving spacetime. Einstein's General Relativity shows that mass and energy distort spacetime geometry, and objects (and light) follow the straightest possible paths (geodesics) in this curved spacetime. For example, the Sun's mass bends spacetime so Earth's orbit is simply Earth "falling" around the Sun. Equivalently, gravity is spacetime curvature: an object feels weight because its worldline is deflected from straight in the presence of mass. In Newtonian terms one can think of a force toward a mass, but fundamentally gravity is geometry: mass tells space how to curve, and curved space tells objects how to move.