Antoine Le Baux, thanks for reaching out.

I guess this is kind of achievable by just having all these as tags already, but it's more of a visual aid and is more of a META thing you wouldn't want to export to file metadata.

Basically it just puts a small flag icon on the preview of the photo or a frame around the photo (see the attached pictures). It can be useful for example when doing aviation photography and uploading to JetPhotos where a picture can be approved or rejected and I can mark it in my library as such. The same thing with colors, it can be helpful when trying to pick out a couple of photos from a larger collection. Both flags and colors are searchable queries and can only be assigned one color at a time and one flag. For example I use the color blue for photos I want to upload in the future, yellow for photos waiting to be approved etc.

Adobe has an explanation on their website as well, just google Lightroom flags and this should be the first link.