Print-Ready Files & PDF/X
Print-ready files are the prepared output of a layout for a printer. PDF/X is a PDF variant laid down in the ISO 15930 series of standards for exchanging print data, in which fonts are embedded, colour definitions are unambiguous and content unsuitable for printing is excluded, so that the result stays predictable.
In practice
Ask your printer three things before exporting: which PDF/X profile they expect, which colour profile applies to the chosen paper, and how much bleed they need. Bleed means that images and areas running to the edge extend beyond the trimmed format so that cutting does not leave white slivers, which is by far the most common cause of complaints. Agree with the printer whether images should already be supplied in the print colour space or whether a PDF/X-4 workflow with an output profile is used, and check that black body text is set as pure black rather than a four-colour mix, otherwise the smallest registration shift makes it look fuzzy. Have the supplier hand over the print-ready file and archive it together with the open layout files. Owning only a PDF ties you to that supplier for any reprint.