Foxconn is Apple's primary manufacturing partner in China, building devices from iPhones and iPads to most of the Mac line. The contract between the two companies accounts for a good portion of Foxconn's revenue, and The Wall Street Journal links Apple's success with the iPhone 6 and iPhone 6 Plus to the manufacturers growth:

The world's largest contract manufacturer for electronics by revenue derives almost half of its revenue from its major client, Apple. The Cupertino, Calif. company sold 74.5 million iPhones in its fiscal first quarter ended Dec. 27, up 46% from a year earlier, lifting its profit to a record $18 billion.

Despite reports that it is looking to grow beyond manufacturing, the relationship with Apple remains very important for Foxconn. The company is currently planning a $2.6 billion display facility for a single client, most likely Apple. Additionally, Apple will reportedly partner with Foxconn for a device trade-in program in China, that will see Foxconn refurbish and resell iPhones through their own channels.

Source: The Wall Street Journal