system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The saying "if it's free you're the product"
gpt
This is usually said about advertising supported services that are free of cost to the end user, such as network television or radio (and since expanded to cover online services too).  When that happens, it is indeed the viewer that is the product.  The television networks produce shows to attract an audience which they then essentially "sell" to their real customers, the advertising sponsors.  Those sponsors want to target specific demographics of people, and the networks do their best to collect that data - which age ranges watch a given show, etc.

So that's the basic idea.  The customer is the advertiser paying to put commercials on the air.  The product is the audience watching those commercials.