Laura, The Clean installer helps you to identify what type of package files are in your downloads. For example, most of the times it gives you the information regarding if the files are Objects (it identify as Objects the files that are furniture and the build items), Recolors, Floors, Walls, Clothing, Meshes of Hair, Hair Recolors, Glasses (Accessories), Skins and so on. Also, if you check on the Preview option, most of the time when you click
the entry, it opens a a pop-up with a preview of the mesh to give you an idea which is the file. You also have the option to choose how to sort the files by clicking in the header.
The files highlighted in RED are hacks and files that overrides the original game ones.
The files highlighted in PINK are duplicate files, so you can delete one of them (with the menu that opens with a right click). If you are not sure if you should delete the file, the same menu also gives you the options to disable or move the file. This is useful in cases when you have a duplicate file that has a name that you can understand and another one full of characters that confuses you. It also helps you to identify if you had an old mesh in your downloads and later you add a file that is an updated or fixed mesh of the original one (as most of them are named like the originals by the persons who fixes them). If that the case, those two files of the same mesh create a conflict because you can only have one of them in the downloads. And you can find out which one is the newest one by the date of installation that the CI gives you. Perhaps these files highlighted in pink are the ones that you need to review carefully, especially if you have too many of them.
You can find The Clean Installer Quick Start Guide, FAQ and the Manual with more detailed and accurate information here.