In physics, the concept of "curved" refers to the curvature of spacetime, which is a fundamental concept in Einstein's theory of general relativity. In this theory, gravity is not seen as a force acting between masses, as in Newtonian physics, but rather as the curvature of spacetime caused by the presence of mass and energy.
In general relativity, mass and energy deform or curve the fabric of spacetime, and this curvature affects the motion of objects within that spacetime. The curvature of spacetime can be visualized as the warping or bending of a flexible two-dimensional surface (like a rubber sheet) caused by the presence of massive objects.
The presence of mass and energy tells spacetime how to curve, and the curvature of spacetime tells matter and energy how to move. Objects, such as planets or stars, move along paths dictated by the curvature of spacetime created by the distribution of mass and energy.
To illustrate this concept, consider the analogy of a ball rolling on a curved surface. If the surface is flat, the ball will move in a straight line. However, if the surface is curved, the ball's path will be curved as well, following the contours of the surface.
Similarly, in a curved spacetime, the path of an object, such as a planet, is influenced by the curvature. The presence of mass or energy creates a curvature in the surrounding spacetime, and objects move along curved trajectories as a result of this curvature.
So, when we say that spacetime is "flat," it means that in the absence of mass and energy, the spacetime is described by a flat geometry, like a perfectly flat, unwarped sheet. But when mass and energy are present, the curvature of spacetime becomes significant, leading to the effects we observe as gravity.