That's how long the average reviewer spends on your resume, before making an interview/no interview decision. That's IF your resume gets beyond a database search.
To put it in perspective, think about what you can do in 15 seconds: Brush your teeth, kiss your significant other, uncork a bottle of wine, pour a beer, load the washing machine, pour coffee, tie your shoes, put on your shirt, tie your tie, or watch Monty Python's Slapping Fish Dance.
So how can you maximize your 15 seconds of fame?
First of all, don't assume that the first person to read your resume is the hiring manager. The hiring manager is busy, and often has someone from HR, or an admin do a visual pre-screening.
Write your resume to grab even a person who's not in your field, and show them you are perfect for the job. Club them over the head with relevant examples, and customize your resume to the specific job. Job seekers who get indignant and feel that the reviewer "should know what this means", end up in the don't interview pile.
Remember your 15 seconds are likely on the screen, so that means your goal is to get the pre-screener to press page down. Put the sizzle above the fold.
Forget big paragraphs about objectives or summaries. You've only got 15 seconds.
So how will you use your 15 seconds...will it be your first 15 seconds with a new company? Or your last?