Akasha is the element from which all things arise and to which all things return.  It is the backdrop or the "space" that makes it possible for all the other elements to exist.  It is all at once completely empty and yet all encompassing.  We think of all that we know as being made up of "stuff", but if you look closely enough, there's space between the stuff.

When I was in Costa Rica earlier this year, studying with Shiva Rea, I was blessed to have a physicist in my tribe.  As a wildly intelligent, spiritual practitioner, he would not only entertain my "nerding out" about all things yoga, but he would translate the vastly complex ideas into simple terms that I could understand and wrap my mind around.  I'll never forget one of our conversations about the space element when he asked me to imagine the smallest possible thing that I could.  I answered with a single atom and he described how it could be smaller still because at the center of the atom is a nucleus made up of protons and neutrons.  (Something I learned years ago, but hadn't thought about in ages.)  The next closest "something" circling the nucleus is an electron in the electron cloud...stay with me.  Between the nucleus and the electrons is space.  Now, to put this into terms that we can more readily understand, imagine that the earth were the nucleus of an atom, the next closest something (an electron) would be at a distance of Jupiter away!  So the stuff we think of as solid is really made up of organized space.  Unless there's smaller stuff that we don't yet have the ability to understand.  Hopefully I didn't lose you there, maybe Dustin Hoffman and Mark Walberg can explain it more clearly:

When hearing the word "space" I generally think of outer space - the cosmos and the sense of awe rushes over me when gazing up at the night sky.  For me, this sensation often results in contemplations on infinity and the edge of the universe which Inevitably yields acceptance of the unknown and joy in the unknowing. 

We can search and seek outside ourselves for answers, but yoga gives us an opportunity explore the inner space of our consciousness with the same curiosity and wonder that we do outer space.

Here's another video of just how mind boggling the scale of space can be.

And finally, here is a sequence to awaken the space element, akasha inside the body.