What is the purpose of "overlays" folder?

When I import a Maven project to IntelliJ IDEA, there is a folder named "overlays" created. I can't find any notes about this folder in the IntelliJ docs. What is the purpose of this folder & its contents?

(There ia a .idea folder and a .iml file for keeping project meta data. I can't figure out the purpose of this "overlays" folder)

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