what exactly do these do?
R.I.P Dimebag Darrel 1966-2004

Gear :

Squier Affinity Strat
Fender Frontman 15G
Boss DS-1 Distorion
These are used largely as an alternative too mic up amplifiers, used in recording and live sound.
Where'd you hear DI box from?
#12 of the "Ibanez SZ over RG Club!" PM ibanez4life SZ! to join!
It stands for "direct input" and it lets you send guitar or bass straight to the mixing board. Usually it includes some kind of speaker EQ response sim.
I'm not very active here on UG currently.
I'm a retired Supermod off to the greener pastures of the real world.