Marmoset Toolbag (https://marmoset.co/toolbag/) is a real-time PBR rendering software and its great for previewing your scene or objects in real-time and near photo-real, its very easy and fast to setup and you immediately are capable of previewing your PBR materials and determine if somethings need to change. It has lots of render features like real-time ambient occlusion, voxel based global illumination, soft shadowing, reflections/refraction, emissive materials, volume fog, DOF, glow effects, chromatic aberrations, etc. It can also play and render animation and import Alembic files.
You indeed need Marmoset Toolbag software on your machine in order to use the viewer, if you have it then go to export and select Marmoset viewer, this brings up a window showing export options, set the preferences and click export, this outputs a .mview file and you can upload it in the section “embeds” below the preview images in the CGt publisher, click the “add Marmoset preview” window and upload the .mview file, that's it.
I use Marmoset allot and I like it because its so fast and easy to setup and it renders amazing quality instantly. I also use it to play with lighting rigs and have real-time feedback on it before I move to Octane Render.