Interleaved learning is an approach to learning that departs from the traditional block or focused approach to learning. Interleaved learning and the research behind it, is based on the idea that if we should not focus on learning or mastering one skill or set of information, such as a limited group of words in a foreign language. Instead , we should move on to other skills and bits of information, and then come back to the first group of skills or information later.
Research shows that, in the short term, the block learner does better. In the long run, the interleaved learner retains more. See some interesting examples.
Here are a couple of videos on this and how this relates to learning languages at LingQ.