When applied to a position property, it places it on the edges of the comp. Use "time" as the percentage input like in the example below to drive the animation.

Move along the edges of the comp - After Effects Expression.jsx

function MoveAlongEdgesOfComp(percent, marginpercent) {
  
 percent = (percent === undefined) ? 0 : percent % 100;
 marginpercent = (marginpercent === undefined) ? 0 : clamp(marginpercent, 0, 100);

 // convert the original percent value to both horizontal and vertical percent values to drive the motion
 horizontalpercent = (percent >= 25 && percent <= 50) ? 100 : (percent >= 75 && percent <= 100) ? 0 : (percent < 25) ? (percent * 4) : (100 - (4 * (percent % 25)));
 verticalpercent = (percent >= 0 && percent <= 25) ? 0 : (percent >= 50 && percent <= 75) ? 100 : (percent < 50) ? (percent - 25) * 4 : (100 - (4 * (percent % 25)));
 midPoint = [thisComp.width * 0.5, thisComp.height * 0.5]; // use the linear function to drive each dimension
 xLoc = linear(horizontalpercent, 0, 100, 0, thisComp.width);
 yLoc = linear(verticalpercent, 0, 100, 0, thisComp.height);
 finalPoint = [linear(marginpercent, 0, 100, xLoc, midPoint[0]), linear(marginpercent, 0, 100, yLoc, midPoint[1]), 0];
 return finalPoint;
}

MoveAlongEdgesOfComp(time * 20, 50)