Imagine you're in a Tower Records in the late '90s. You head for the cash register with a credit card and two compact discs in your hand. Let's say one CD is by OutKast, the other by Smash Mouth (remember, it's the late '90s). The following week on the Billboard 200, America's premier album chart, both of the CDs you bought have been tallied. In effect, by plunking down 16 bucks per disc, you and thousands of fellow Americans gave each of these albums a "vote" on the chart that week.
Now let's flash forward three months: You're playing the OutKast CD constantly, but the Smash Mouth disc is gathering dust, having been played maybe once or twice. How is this activity reflected on the Billboard charts?
Simply put, it isn't: The album chart doesn't reflect what you and your fellow buyers did with those discs after you acquired them. Sure, you could tell a friend, "Hey, I'm really loving that OutKast album," and perhaps that will inspire your friend to go buy the CD, too, helping to keep it aloft on the album chart. But that's a pretty indirect way to reflect your abiding love of that album. Three months ago, your trip to Tower Records gave equal "votes" to a pair of CDs you now feel very differently about — one vote each for the OutKast album you adore and the Smash Mouth CD that's now a beer coaster.