What does it mean by 'Instant preview for this file type not available'?

For certain file types like PDF, DOC, DOCX, EPS, CDR, AI, PSD, TIF, TIFF, PPT, BMP, SVG, our system is not able to generate an instant proof for review but we have your file and our pre-press department will review your file for size and quality (resolution) before sending it to the production. They will contact you if the file is not good for printing. If a proof is requested, you will receive an email proof (even if the file is good) within 24 hours and we will only process the order after you review and approve the proof.