Arrow spine refers to how stiff the arrow is. To determine the spine a shaft is rested on two points 28 inches apart and a weight is hung from the center of that 28 inches. The amount of bend in the shaft determines the spine of the shaft.

A 300 spine arrow only flexes .3 inches where as a 500 spine arrow flexes a half inch. The more it flexes the weaker the spine.

The length at which you cut the arrow and the amount of weight at the tip and/or rear will also determine how stiff a certain arrow acts when actually shooting it from a bow. For instance, say you have two 400 spined shafts. You cut one of the shafts at 30 inches long and the other at 27 inches long. The longer shaft is going to act weaker than the shorter shaft when fired from a bow. If the 30 inch long shaft with a 125 grain tip reacts correctly for a particular set up then you would have to add weight to the front of the 27 inch long shaft in order to get the same amount of flex in the shaft when fired from a bow.

Also, disregard the numbers that are printed on most arrow shafts today. Most of them do not refer to the actual spine of the arrow shaft.

I hope this helps and wasn't too confusing.

As far as what you should be shooting I agree with BC.