How to overlay in AEM
AEM offers two ways to customize OOTB resources:
1. Overlays which allow you to re-define existing resources (i.e.: replace current behaviour).
2. Overrides which allow you to extend existing resources (i.e.: add new behaviour).
This article assumes you have already used overlays and will discuss some of the best practices for for doing so. If you are not clear on how overlays work or how they are different from overrides, I recommend this article from Adobe or this helpful comparison of overlays and overrides from aemvardhan which includes a quick tutorial.