The term is used to describe a methodical process used to ensure that a digital file created by a client and sent to a printer or service bureau is set up correctly. There is professional software available that can be customized to pre flight files. In printing terminology 'artwork' is anything created for printing.
Some of the steps in preflighting artwork are:
• making sure that all images are 300 dpi or better
• ensuring that all images are CMYK
• ensuring that all typefaces (fonts) are sent to the printer with the project file, or
• all typefaces are embedded in the project file, or
• all type is converted to outlines (this makes it impossible to edit type later)
• if any graphics extend to the edge of the printed page, projects must account for bleed
If you keep these things in mind when designing the project, you can basically preflight as you design. Nowadays, most layout programs (Quark Xpress, Adobe InDesign) include preflight functionality. Also, since many printers now take high-resolution PDF files for projects, the PDF exporting process lumps much of this preflighting into its export engine... you just have to thoroughly examine the PDF export summary before the final export.