Based on an item level and how rare it is (green, blue, purple, etc.), each item has a limit as to how much can be added.  Each stat point or effect costs points to be added to the item.  This means an item can have 10 int or 10 str, but not both.  You might be able to have 7 int and 7 str because each stat point of the same type costs more than the last.  +spell damage, +attack, +to hit, and so on work in the same way.

This calculation is used to determine what is the best item for you, based on what you think is important.  For example, a rogue has no use for int, so he would sent +int to 0.  The same rogue will set agility to 2 but strength to a 1 which means that he wants agility first, but if the item has enough strength he might use it.  That's the general idea.

It can get a lot more complicated for some classes, so I would suggest setting the stats you know you don't need to 0 and leaving the majority of the others alone.  Obviously, a rogue would bump up agaility, a mage would bump up Int, etc.  But if you don't know if you want +spell damage over +crit, it is generally better to leave it at the base values.