The phrase "big data" refers to the idea that large databases of seemingly random data about people is valuable. Retailers save our purchasing habits. Cell phone companies and app providers save our location information.
Telecommunications providers, social networks, and many other types of companies save information about who we talk to and share things with. Data brokers save everything about us they can get their hands on. This data is saved and analyzed, bought and sold, and used for marketing and other persuasive purposes.
And because the cost of saving all this data is so cheap, there's no reason not to save as much as possible, and save it all forever. Figuring out what isn't worth saving is hard. And because someday the companies might figure out how to turn the data into money, until recently there was absolutely no downside to saving everything. That changed this past year.
What all these data breaches are teaching us is that data is a toxic asset and saving it is dangerous.