Cats' eyes is the name of the small reflective devices placed on the road so drivers can see lane divisions in the dark. (At least, that's what they're called in Ireland). They get the name from the way in which they appear to glow unnaturally in the dark, like cats' eyes.

Normally, white cats' eyes are put in the middle of the road, dividing the traffic going in different directions (where the white dashed line is), and orange cats' eyes are put between lanes going the same direction and between the outermost lane and the side of the road (where the orange lines are). (I believe the colours are reversed in the U.S.A..) In addition, about 10 to 20 cats' eyes on the outside of the road are green where another road intersects (which is too small to be worth putting in a traffic light (stop light)), this allows the driver to know when to use the indicator (turn signal), allows enough time to slow down, and easily identifies exactly where the intersection is (the cats' eyes return to orange immediately after, so it's always between the last green cat's eye and the next orange one).

This means there are 3 types of cats' eyes: white on both sides, orange on both sides, and orange on one side but green on the other. The orange/green ones must be oriented correctly so oncoming traffic always sees the green side before and the orange side after, no matter which direction the traffic is coming from. (Rarely are two intersections so close together that green/green cats' eyes are necessary.)