little stop sign to the side of some images' labels

Hi,  I've misread the symbols a little here.  Clearly the image I posted relates only to star ratings.

The images that I transfer to Lightroom are always Nikon RAW files; mostly they always import successfully.  In this instance a small number of photos are being refused import my Lightroom.  They have not been imported previously, which might stop an import, so I'm not sure why Lightroom would not allow the import.

As EXUSA stated, this is for setting how many stars (1–5 ratings), the "negate" symbol on the left is to remove any stars you may have given an image. 

From Bridge, you can open an image into Adobe Camera Raw for editing. You can open any TIF, JPG, or any raw format into ACR. You cannot open any TIF with layers (they must be flttened before you can open them in ACR, plus you must purge the cashe for that folder to update the cashe so that Bridge can recognize that the file is now flattened).

You cannot import GIF images into ACR, but you can import TIF or PSD with or without layers into Lightroom as well as PNG images. 

So, what kind of images are you having issues with, and can you please clarify if you are using Lightroom Desktop or Lightroom Classic.

A stop sign is red and says stop. I'm not seeing one in your screenshot. What you posted is how you set the star rating for a file.