5x100 is the bolt patten that means that there's 5 lugs that's 100mm apart.
The easiest way to explain offset is that it's how close the wheel is to your car, the higher the number, the closer it is to your caliper/hub, the lower the number, the furthur it is away from your car. Why is it important? Because you need to clear your calipers (prevent the spokes from hitting it) and it make sure that your tires don't rub.
For most cases you can run +28mm to +44mm offset on the Celica, stock is about +39mm.