Google's Native Client (NaCl) is a brilliant idea. TL;DR: it allows native code to be run securely in the browser. Here's a collection of developer-centric notes about it (it's not very Unity specific).
But is it secure?
"Bububut, waitaminnit! Native code is not secure by definition" you say. Turns out, that isn't necessarily true. With a specially massaged compiler, some runtime support and careful native code validation it is possible to ensure native code, when ran in the browser, can't cause harm to user's machine. I suggest taking a look at the original NaCl for x86 paper and more recently, how similar techniques would apply to ARM CPUs.
But what can you do with it?