EditorConfig is a dotfile that enables automatically setting text file options such as "space or tab indentation", "indentation size", etc.
In Tuleap, we use most of the time 4 spaces, except on .tab files for example. This means that if you set your editor with a basic setting of 4 spaces, you will get an error after editing a .tab file because the editor will have inserted spaces instead of tabs.
EditorConfig lets you define such configuration based on file extension and even filename.
It is automatically recognized by many text editors and IDE, and there are plugins for most editors.

We have used it for some time in angular apps and a few newer plugins. We should set a base file at the root of the project and only add sub-configurations where needed. E.g. angular apps which use bower.json or package.json files, which can be automatically generated and enforce 2 spaces.