When it comes to arcade games a flyer is a promotional brochure. Well, actually a flyer is a promotional item concerning just about anything, but this writeup deals with arcade flyers specifically.
Flyers are usually mailed out by the game manufacturers, and passed around at trade shows to try to get operators to purchase the game in question. Most long-term game operators have thousands of assorted flyers that they have accumulated over the years.
As the years have passed, flyers have become a valuable resource in determining what certain games looked like, and what configurations they were available in. Some games simply don't exist at all anymore, save for a flyer or two.
Flyers have recently become collectable. Many people try to get the flyers for the games that they own. Some people even collect flyers just to have them.
There will often be multiple versions of any given flyer. Sometimes a company will make flyers for games that never even end up being produced. At the moment, most flyers tend to sell for under $10 USD, and most of them can be had easily on eBay.