Hard mods are modifying the 3DS hardware, whereas soft mods are software. The difference is doing something like installing a chip into your 3DS (hard mod) or leaving it with the original hardware and only modifying the operating system to allow third party apps or modifications to the first party apps (soft mods)

I was intending to play through that soon, I’ll have to keep that in mind before I do.

Hard mods or soft mods?