DARPA is working on a new method of learning called Targeted Neuroplasticity Training, which they are saying could lead to "downloadable learning". The concept sounds simple enough on paper; peripheral nerves are activated selectively and precisely to trigger reactions in the brain, with the end goal of mimicking long hours of learning certain tasks. It's obviously quite a bit more complicated than that, but the basic idea is that knowledge and experience could theoretically be broken down and analyzed to generate a comprehensive, brain-deep training program. DARPA wants to use the system to accelerate training for military personnel who have to undergo extensive training, such as translators and cryptographers.
To go a bit deeper, stimulation to the peripheral nerves can help to control the release of neurochemicals like acetylcholine, dopamine, serotonin, and norepinephrine, which control neuroplasticity. Neuroplasticity, for those not in the know, is the brain's ability to forge new neural connections; this means not only learning certain things, but changing the way that the brain learns in general. This means that even if DARPA doesn't reach their theoretical goal of being able to download a full learning regimen to a receptive brain, they could still trigger high-grade neuroplasticity of the sort normally only found in children, and train personnel far more quickly and effectively than through the usual means. While this sort of program will obviously require things like implants and surgical stimulation at first, DARPA announced that they will also be working on non-invasive methods of inducing the signals and stimulation that they need in subjects. This program is part of a set of similar programs out of the White House, called the White House BRAIN initiative.
They're not the only ones looking for non-invasive ways for data and the human brain to interface. Facebook's elusive Building 8 team, coincidentally headed up by DARPA alumni Regina Dugan, is currently working on a thought-controlled text system that could make typing and voice text entry obsolete, and possibly expand to other methods of controlling a digital system. Elon Musk and his people are also working on a non-invasive brain interface, though experts are saying that such technology is still quite far from being completed.