A DI box (direct input) is an impedence converter for getting signals off the stage and into a snake for the run to the board. This particular one will also take a powered speaker output.
Basically, it converts signals into balanced XLR connections that you can connect to a snake and make a long run to the board without a lot of loss.
Is google not working today ?
Snake = a long multicore cable that connects from the stage area back to the mixer desk, which is usually in the middle/rear of the hall, each instrument has a channel in the snake
Loss = loss of quality of the audio signal when passing through anything, it is lost by both losing voltage due to resistance/impedance of the cables, and by spurious external unwanted 'noise' signals creeping into the signal you put in at the stage end