What means "Objects" in Apple Photos?

Then search for audio files or photos you edited in an external editor.

Or search with a smart album based on the file type:

What is the image file format of your photos? If the filename extension is ".jpg" try the rules

Match all:

• photo is not video
• filename does not include .jpg

If your images are raw files, search for the extension of your RAW format instead.

Objects are media items, that do not qualify as either video or photo. It could be Audio file, PDFs, images or videos in an unsupported format.

If you migrated an Aperture library or an iPhoto Library to Photos, the library may contain items that are not compatible with photos.

Try to find these items with a smart album:

• File > New Smart album
• Add a rule: Photo is not able to upload to iCloud Photo Library.

Is this album empty?

See this page: Check the status of your library in Photos for macOS - Apple Support

Indeed. The smart rules could be improved. For example - I can create a rule "Photos is/is not video", but a rule "Photo is not photo" is missing. That is not very consistent.