Because in general relativity, gravity is not a force but a curved space-time, if a particle is not stressed except gravity, it is regarded as a free particle, and its trajectory is geodesic with covariant derivative of tangent vector of 0, which is also commonly known as straight line.

The difference is that if the Riemann curvature tensor is zero in the absence of gravity, then the covariant derivative in a coordinate system is its ordinary partial derivative, so the geodesic becomes a real straight line. If the Riemann curvature tensor is not zero under the action of gravity, and the covariant derivative differs from the ordinary partial derivative by the Kirkhner sign term, then the geodesic line is no longer a straight line at this time, which is also the action of gravity, which changes the nature of the straight line instead of acting on the object as a force.