Acid-base chemistry is one of the more complex aspects of wine/mead making, and you're jumping in on your first batch ;D
Acid blend is composed of malic, citric, and tartaric acids in various ratios depending on the manufacturer.
Acids dissociate when dissolved in water to yield positively charged hydrogen ions aka protons (which cause the pH to drop) and a negatively charged "conjugate base." If they dissociate completely (like sulfuric acid) they are said to be strong, and if they dissociate incompletely (like the three acids above, as well as nearly all organic acids) they are said to be weak.
Weak acids have a property called pKa, which is defined as the pH at which the acid is half in the dissociated form. The lower the pKa, the stronger the acid.
The pKa values most relevant to mead making are
Citric acid: 3.08
Tartaric acid: 3.2
Malic acid: 3.4
Gluconic acid: 3.6
Gluconic acid is the primary acid in honey, produced by the action of a bee-secreted enzyme on glucose. Malic acid is dominant in apples, pears, and quince, both malic and tartaric acid are prominent in grapes, and citric acid is prominent in citrus, though most fruits have some level of all three.
Now to get around to your quote: total acidity is the sum concentration of all acids in the mead. Adding calcium carbonate neutralizes acid as follows: The carbonate grabs a hydrogen ion in a reaction that ultimately produces water and CO2. The calcium ion remains in solution along with the conjugate base of the acid. Because lower pH = more hydrogen ions, taking them away will raise the pH.
Let's say your pH is 2.6 and you start adding calcium carbonate. Once it rises to around 3.08, the pKa of citric acid, you start pulling protons off of citric acid, yielding calcium and citrate ions. At 3.2 you neutralize tartaric acid, at 3.4 you neutralize malic acid, and at 3.6 you neutralize gluconic acid. Once the pH is up around 4 you have neutralized pretty much everything, resulting in a bland, "flabby" acid profile.
Tartaric and citric acids are a bit special, because the neutralization products, calcium citrate and calcium tartrate, are insoluble particularly at cold temperatures and will precipitate out. Calcium malate is partially soluble and can precipitate over long periods but generally stays in solution. This is important because calcium ions, like sodium ions, trigger our "salty" taste buds and thus impart a detectable flavor above a critical concentration.
Solubilities:
Calcium citrate 0.9 g/L (0.09%)
Calcium tartrate 0.4 g/L (0.04%)
Calcium malate ~9g/L (0.9%)
Calcium gluconate 33 g/L (3.3%)
The reason they recommend not to reduce acidity more than 0.3 to 0.4% is because that is typically the amount of tartaric acid present in wine. Once you add more than that you start neutralizing malic acid, resulting in free calcium ions and potentially a salty taste.
If you don't add fruit or acid blend, the primary acid present will be gluconic. Since calcium gluconate is highly soluble, it is possible to create a salty/chalky flavor by overdosing with calcium carbonate. However, given that the total amount of gluconic acid in honey is around 0.5% and this is diluted to around 0.12% when making mead, the amount of calcium carbonate required to keep the yeast happy is generally below the taste threshold.
In short, you'll probably be fine. Don't be surprised if you see some tartrate crystals (which are harmless). Depending on the amount of malic acid you neutralized you may have a slight salty taste, but most likely barely enough to notice.
"Calcium carbonate reacts preferentially with tartaric rather than malic acid, so one should not try to reduce acidity more thab 0.3 to 0.4% through its use. A dose of 2.5 grams per gallon of wine lowers TA about 0.1%. After its use, the wine should be bulk aged at least 6 months to allow calcium malate, a byproduct of calcium carbonate use, to precipitate from the wine. The wine should then be cold stabilized to ensure tartrate crystals do not precipitate out after bottling."
