[alexiskai]

To add more context to D.J.'s reply, the central idea with the spring pressure is that the spring needs to clamp the valve face to the seat when the valve is fully closed – both to keep it from leaking and to facilitate the sinking of heat from the valve into the block. There's a certain target pressure needed to do that.

You need to make sure your spring will deliver that target pressure at a certain length. That length is the "installed height," the total distance between the ends of the spring when it's installed in the valve chamber. It'll be pressing against the retainer at the bottom, and against either the shoulder of the valve guide or the block itself, depending on the type of valve guides you have. This variability is why you should take the length to be tested from your actual engine instead of from a book.