Static spine is the measure of deflection that an arrow has with a 2 lb weight hanging from the middle with end supports 28" apart. For instance, an arrow that says it's 340 means that it deflects .340" with the weight hanging from it. The lower numbers are from arrow that deflect less and are therefore stiffer.

Dynamic spine is the actual deflection that an arrow has when shot out of your bow. Only a relatively small range of defection will fly properly out of a given setup. You want to tune your dynamic spine to match this range. Shortening arrows increases dynamic spine (stiffens them). Lowering draw weight also increases dynamic spine, and lowering tip weight does the same. The reverse of these will weaken spine. Spine tuning an arrow means to adjust these in combination to obtain a good flying arrow for your setup. The logical way to start is to look at the tip weight (FOC) you desire and then work from there. You can easily shoot arrow designed for 90 lb draw weights out of a 60 lb bow, if you know how to weaken the dynamic spine properly.