just like people above me said..
it is a tool for update luma
the usual process is:
turn off the console, remove sd card, put on pc, go to git hub, download the zip file, extract the file to sd card, put back on 3ds..
alternativaly you can use a ftp client but in some cases or when you just don't want to use another device ftp is not very lazy friendly
i mean, for really lazy people like me, or someone who isn't on a pc at all time having the possibility to update on the console itself, it saves a little of work...
specially when luma3ds is updated a few times a week (not complaining) and some of us always get hyped to update it
however, we shouldn't have a luma update app...
wee should have an homebrew update utility so i can update not only luma but most usual homebrew and apps..