Special relativity explains that being inside a plane going at a constant speed is like being stationary because time and space adjust to keep it so. General relativity extends special relativity to attribute the acceleration caused by gravity to changes in space and time as well.
QR5.4.1. Free-fall is Acceleration
QR5.4.2. The Gravity Gradient
QR5.4.3. Gravity And Photons
QR5.4.4. Gravity Bends Light
QR5.4.5. Gravity Slows Time
QR5.4.6. Black Holes