broadband noise is basically are extraneous sounds you computer may make that may show up while recording to your sound card...pings to your modem/router...power supply fluctuations, opening your disc drive, or even clicking on the mouse can intruduce small amounts of white noise to your recordings. you could also get this noise if you or someone else turns on anoisy device like an oscilating fan, air conditioner, blender coffee grinder etc near by the computer that is recording.
you may hear the broadband noise if you are monitoring the audio while recording, but as john says, if it doesnt show up on the finished product then dont worry about it.
i got a separate power conditioning power strip (furman) to deal with the exterior noise on my transfering computer, but the broadband noise interior to a computer is harder to deal with.
hope this helps