Interleaving is the admixing of practice with closely related problems. The challenge is that you want the problems to be from the same domain of knowledge but not so closely related that they are merely variations of the same problem. An example where interleaving is not practiced and is a problem is in mathematics texts which tend to have chapters devoted to one type of problem at a time. When readers work on the problem sets at the end of a chapter, they are basically working to solve minor variations of the same problems that the chapter taught.
With interleaving, we're striving for a scenario where the learner is faced with a set of problems from the same domain of knowledge BUT we want them to be challenged with each problem to answer the question, "What kind of problem is this?" rather than to move rotely through a series of problems applying the same formula or solution to each one.