Camber is how much the wheels point inwards or outwards. If you stand at the rear of the car and look at your wheels you can see the bottom of the wheel slightly pointing out. Think of those "flush" car looks. Notice the wheels bending outwards?
e.g.
Attachment 17327
When you lower the Accord Euro the camber will become more negative meaning the bottom of the wheels will point out a bit more (not as exaggerated as the pic though). Getting the camber kit helps the person doing the wheel alignment to adjust your car to within specs (since they can't adjust the camber without it). The camber kit will help reduce excessive wear of the inner tyres but if you're not concerned just rotate your wheels regularly and wheel alignment.