It means what it says
a) you used a font that does not allow embedding in PDFs
b) you used spot colors (Pantone et al) without converting them to process CMYK
c) you didn't check the overprint/ trapping
Most of that no doubt could easily be checked using the prepress toolset in Acrobat on your end as well. As for whjat the specific fixes are, nobody can tell based on your vague info. We would have to see screenshots or the resulting product, know what fonts you used, require system info...