Physics says, spacetime is curved. This sounds mystical, but it's simple.
Nothing is ever straight because nothing stops moving.
We're told the shortest distance between two points is a straight line. But if an airplane plotted a course between two cities in a totally "straight" line, it would be a crash course – a straight line between two points on the earth goes underground.
Airplanes and cars and people all have to follow the surface of the earth. When we are traveling, we never notice. The earth appears flat locally, curved globally.
Spacetime is like this too. The shortest distance between two points is a curve.
Imagine throwing something at a target. You have to aim a little higher than the target, to account for the projectile dropping while it travels. You've heard the term called "escape velocity"? To launch something into orbit, you just have to get it going fast enough so that when it drops, it falls over the horizon. Orbit is just falling over the horizon indefinitely.
Staying in orbit takes almost no energy. Changing orbit – up or down – takes huge amounts. The shortest distance in orbit is staying in orbit (which is a curve).
The straightest thing possible should be a beam of light, but light is attracted by gravity. All the photons in a laser will eventually curve into some orbit.
Space and time and motion can't be discussed separately, and neither can gravity and energy.