Preflight in the printing process basically means to check a document or any print project for technical errors to make sure the project is ready to be printed.  The preflight process involves multiple steps including running the file through preflight software.  The software checks for font conflicts, misused colors, missing or broken pictures and graphics, or any other things in the document that might cause problems or the printer.  Indesign has a built in Preflight application to make the production of files easier but when using Photoshop or Illustrator, Preflight must be done manually.


 Preflight Checklist:

• Extend bleed artwork to bleed lines
• Check that important content is within safety margins
• Check and edit colors
• Check that process colors are CMYK (not RGB)
• Check that spot colors are converted to CMYK or, if desired, carefully defined and applied as spot
• Trash unused color swatches
• Check and edit fonts (with the help of Quark's "Usage" menu item or Adobe's "Find Font" menu item)
• Make sure there are no missing or inactive fonts
• Check that fonts are styled in their "true" form (in other words, unaltered by artificial "bold" or "italics" options in the application)
• Check that linked graphics are up to date
• Tidy by deleting unused elements, elements on the pasteboard, and empty boxes
• Check layers
• Make sure that the template layer only contains template elements
• Template layer is set to non-printing or turned off
• Artwork layer only contains artwork
• Create a color proof to check your document for color quality, spelling and layout.
• For an InDesign layout, simply open the document and select "Preflight" from the File menu.
• Quark lets you "Collect for Output" from the Quark menu but not preflight. To preflight a Quark document you must use separate preflight software such as Markzware's FlightCheck.
• Preflighting will result in a report listing conflicts and errors with your files. This is normal. Part of preflighting is fixing errors and running it through again until you get a clean report.

A Prepress Technician formats and proofs text and images submitted by designers and clients into finished pages before they are printed.  On average, a Prepress Technician makes $37,970 annually or $18.26 hourly.  

Sources:
http://inkd.com/earn/prepress
http://www.universalprinting.com/preflight.aspx
http://inkd.com/earn/prepress
http://www.bls.gov/oes/current/oes515111.htm