Luma3DS is a custom firmware (CFW) which disables restrictions on the 3DS operating system. This means it allows running unsigned code (homebrew software – including emulators, backups of official games).
In addition to that it introduces some additional features like screenshots, cheats (don't do that online!), blue light filter, force New 3DS extensions (faster processor, L2 cache) on older games.
A good overview can be found at it's wiki: https://github.com/LumaTeam/Luma3DS/wiki
To install boot9strap (B9S, bootloader) please refer only to https://3ds.hacks.guide/ and avoid video or YouTube guides.
If you have any specific questions or problems, just ask.
@KleinesSinchen , love your sig. that's definitely true. I lost an hdd for the wii, 360, and ps3 (3 separate hdds) somehow. since then, I've been the backup king. I invested in 80TBs, and I have all of my smaller hdds backed up 4 times. I've become that paranoid. I think everything is about 14-15TBs, but I bought 8 10TB hdds for all of it, and I have four of them plugged in to my laptop at this very moment to monitor. I'm afraid that if I don't make sure nothing is wrong, they'll fail without me knowing about it at some point or something like that.