The toComp and fromComp functions of After Effects expressions can be confusing - but they don’t have to be! Watch this 1-minute tutorial to understand how they really work and how to use them. An an example, we link the center of a circle effect on one layer to the center of a circle on a second layer such that they always stay in place no matter how you move, scale or parent any of the two layers.

Expression used in the Tutorial

var posInOrangeLayerSpace = thisComp.layer("orange solid").effect("Circle")("Center"); 
var posInCompSpace = thisComp.layer("orange solid").toComp(posInOrangeLayerSpace); 
var posInGreenLayerSpace = thisLayer.fromComp(posInCompSpace);