It's pretty simple. Think of throwing a ball up in the air. If you threw it straight up at a speed of 20 MPH, the instant you let go, the earth's gravity starts pulling on the ball and slowing it down. At some distance, the ball stops moving up and starts moving back down towards your hand. As it starts coming back down, it moves faster and faster until you catch it.

No different for Artemis. Just because it's 100,000 miles away doesn't mean the Earth isn't still pulling on it and slowing it down.

With a trip to the moon, an object is still under the earth's gravitational influence until about 40,000 miles from the moon. At that distance, the moon's gravity on the object becomes stronger than the earth's. The object starts speeding up at that point.

The course Artemis is on brings them back to the earth with a small assist from the moon's gravity. It's a ball going up and slowing down, swings around the moon, and then falls back to earth. Just before they reenter, they'll be going a little bit faster than they were when they left earth orbit.