A print-ready file is a design file that meets all the technical requirements a printer needs to reproduce it accurately — correct bleed, CMYK (or Pantone) color mode, sufficient resolution, embedded or outlined fonts, and the correct final dimensions.
A file that looks correct on screen can still fail these requirements — an RGB file sent to print, for instance, can shift color noticeably once converted, and low-resolution images can appear blurry once printed at full size.
Professional designers typically prepare a separate print-ready export from the working file, rather than sending the same file used for web or presentation purposes directly to a printer.