What does it show?

It shows the quality of images found in the PDF in dpi.

How to interpret the result?

quality of image:
over 300 dpi: appropriate
250 – 300 dpi: good
150 – 200 dpi: mediocre
75 – 150 dpi: low
1 – 75 : wrong

Resolution of images

What does the warning mean?
Resolution of image in dpi
Image, quality, dpi
Images having resolution below 300 dpi may be found in the PDF, which is not appropriate in terms of printing.
How the problem could be resolved?
By modifying the document.
Unfortunately, the problem can be resolved only by replacing the image or by reducing the size of images.