FoundationDB specialized in databases using NoSQL, a way of storing and crunching data without using a relational structure. The company also specialized in ACID-compliant transactions, making FoundationDB the kind of company that could improve Apple's iTunes, App Store, iBooks Store, Mac App Store, and other iCloud-related services.
TechCrunch noted that FoundationDB previously touted:
At current (December 2014) AWS (non-spot) pricing and including enterprise FoundationDB licenses for all 480 cores with full 24/7 support this mega-cluster only costs about $150/hr. In that same hour this cluster will achieve 54 billion writes, yielding a cost-per-write of 3 nanodollars. Said another way, FoundationDB can do 3.6 million database writes per penny.
Apple's boilerplate response was, “Apple buys smaller technology companies from time to time, and we generally do not discuss our purpose or plans.”
There are a lot of things that come to mind when you think about Apple: iPhones, iPads, Macs, Cars, Watches…but the reality is that Apple is one of the largest cloud services companies on the planet. Apple also conducts an incredibly large number of financial transactions through its online stores.
And that's just the stuff we know about. It's conceivable that FoundationDB was purchased to help Apple develop some product or service we haven't heard about yet. The smart money, though, is on Apple's financial transactions.