Okay, here it is in semi-technical brewer-speak.
There are a variety of points during the brewing cycle that gravities are taken. Usually homebrewers only worry about two: OG and FG. There are more, and they tie in with how and why things happen the way they do in the brewing process.
There are "kettle gravities", which allow us to observe - and therefore tweak if we need to - what's going on in kettle. The pre-boil gravity helps us to predict if we're going to hit our post-boil gravity. Kettle gravity determines hops alpha-acid
utilization, or the degree to which the alpha acids isomerize into solution in the wort.
It is a rule of thumb that the lower the kettle gravity, the better the utilization, though there is a lower limit to that rule. It appears that below a certain gravity utilization starts to drop off, indicating that the hops seem to want a certain amount of malt in solution. No one knows exactly why.
Here's a practical example:
Let's imagine a 5 gallons (US) batch of homebrew with a desired of OG 1.048 (12°P). We want that beer to have IBU of ~30.*
When you boil the entire volume of wort, plus an amount to account for losses during the boil, the IBU calculation does not change. Many homebrewers practice a smaller boil volume than the entire brew length, however, say 3 gallons for a 5 gallon batch. If you put all your extract in at the beginning, that can present utilization problems, because the kettle gravity will change if you keep the same amount of fermentables and reduce the volume.
In our example, I'm using 6 lbs of dry malt extract plus some specialty grain. If I add all my extract to a 3-gallon boil, my kettle gravity is 1.095 and the IBU imparted at the end of the boil will be ~19. If I keep 3 lbs of DME back to add late, my kettle gravity is reduced to 1.052 and the IBU will be ~30, right where I want it.
Here's a halfway decent visual reference:
As you can see, the Utilization % is influenced by two factors: Kettle gravity and time in boil. The higher the kettle gravity (represented by the numbers on the right of the chart marking lines), the lower the utilization %, and therefore the less bitterness gets in your wort.
Make sense?
Cheers,
Bob
* IBU prediction, even with an immense weight of brewing science behind it, is not a foolproof, absolutely accurate process. But since neither you nor I can taste the difference between 30 and 34 IBU, practically speaking it doesn't matter much.