Outline
Add an outline on all objects of the layer having the effect. This won't work well if shown on top of the background color of the scene. Be sure to use an object acting as a background or a floor.

Use the thickness property to set how wide the outline is, and color to change its color. Applied on a single object, this is a common way to highlight an item that can be picked up or interacted with.
Note
This is a 2D effect. It has no effect on 3D objects or 3D layers.
Reference
All effects are listed in the effects reference page.