"Everyone knows" what sandbox means. Except that it was a term 'ported into table-top role-playing from another medium and adopted as fashion rather than substance, so no, I don't. Talk is cheap, whiskey costs money, and let's see if this is a thing, and if so, how many things, and if so in a given case, whether it's something you really want to play in.
I've worked on text for this for a long time, as a sequel to my essay Setting and emergent stories from 2011. But it was much more fun to do as a lab, joined by Jason D'Angelo and Ángel Jiménez. Check the games and tags lists for the texts we brought, and feel free to bring up more in the comments. I'd really like to see this topic keep going there.
Video capture didn't fly too well this time, but I went to some effort to make it a genuine audio-visual presentation - as witness the Premiere file graphic:
So when you get the chance, please watch as well as listen. (Also, I promised a couple of file attachments, I'll get them in here soon.)