What does it show?

Is there any crop size and bleed surface of pdf.

How to interpret the result?

If the Trim value is not 0 x 0 mm, then crop size exists.
If the crop size of pdf is smaller than its full size (e.g. 169 x 231 mm => Trim: 148 x 210 mm), then pdf has bleed surface.

PDF bleed and crop size

What does the warning mean?
Pdf has no documented crop size. In such case usually no bleed and crop mark exists either.
Pdf printing office online checker
How the problem could be resolved?
The modules shown below provide crop size, bleed and crop mark to the pdf.
Preparing crop size bleed crop marks
Preparing crop size bleed crop marks