From the site, it seems to be a Web version of mc on JS base, making file management outside of trusted ssh devices less of a nuisance. Fairly straightforward imho.
What is mc?
I guess this is my Ron Burgundy moment - 'I've never heard of it'.
Looks like it would be very useful for some of the stuff I do too. Clearly I've been living under a rock.
Midnight commander (and more generally an orthodox file manager) is especially useful for moving files from one system to another, as it usually supports browsing to remote file systems over ssh or ftp, and gives you a simultaneous view of where you're moving files from and to on the same screen.
The Wikipedia page is a pretty interesting read, actually: https://en.m.wikipedia.org/wiki/Midnight_Commander