. . .ken dude, you suck at googling man. seriously. wat the heck did you search for?
and you've obviously seen too few DIs, i'm guessing 2 only so far (the two that the school has, that's all right).
yea like what colarndo said. it converts and unbalanced signal to a balanced one. unbalanced basically means it's one wave signal. balanced usually means two wave signals that are phased, i.e. direct opposites. one wave is then inverted at the end, e.g. at the mixing desk. any noise picked up will be canceled by the simple law of math that a wave of value +1 is canceled out by a wave of value -1 to a value of 0.
but u don't have to know all this. just know u need a DI in order to plug into a mixer.
and to add, Bass DIs usually have an equaliser, which is useful, and bassists bring one around even if they're not plugging into the house because of the EQ function.