The Csound for iOS API, by Steven Yi and Victor Lazzarini, provides an iOS interface to the Csound language, essentially allowing you to integrate Csound into the iOS ecosystem in order to build applications that use it. This is helpful for a number of reasons, not least of which include that while Apple's Core Audio API is famously well-designed and very high-performing, this is matched in equal measure with its notoriety for being difficult to parse, very poorly documented, and ultimately inaccessible to those without considerable background and time.
While Apple does provide higher-level APIs (especially notable is the more recent node-based AVAudioEngine API), these are often offered at the expense of control and flexibility, and of course low-level access is useful for other things. Csound for iOS puts a wide variety of DSP, synthesis, and more audio algorithms at the fingertips of the developer who either has attained some familiarity with Csound or is willing to. Having used the Csound iOS API for a while now for various projects, updated it, and helped to write materials (and acted as TA) for a course on developing iOS applications with it, I've written and compiled various items of relevance to it. I've described and linked to some of them below. This page also acts as a resource page for EP P453: Audio Programming for the iPad in the Spring 2018 semester, for which I am currently slated to be the instructor.