Mulle-objc ABA improves multithreading performance. But you need to know a lot before you can understand it. Here is what you need to know.
First you need to understand multi-threading. Here is the wikipedia article on mulithreding.
Next it is good to read the Wikipedia article on the ABA problem.
And since you probably want to know how multithreading on Mulle Objective-C differs from multithreading on Apple's Grand Central dispatch.
And you probably want to be aware of the problems in keeping the caches of data consistent.