This term means exactly what it sounds like. It means breaking up nutrient additives into smaller additions and dosing your must over a period of time.
This concept has also been referred to as Nutrient Addition Schedule (NAS).
This is not a required aspect of mead making, but rather an improvement designed to make better mead faster.
Mead must is, of course, so nutrient deficient that mead makers have to supplement the must with a variety of substances, particularly yeast assimilable nitrogen (YAN), to give the yeast healthy enough conditions to ferment honey without producing off flavors. Yeast, unfortunately, aren't so great at rationing their supplies and enter a "feeding frenzy" in the sudden presence of abundance, causing the speed of fermentation to increase. As fermentation is an exothermic process, this causes the temperature of the must to temporarily spike. The end result of a warmer ferment is an increase in the production of fusel alcohols which are responsible for the "hot," "jet fuel," or "young" taste in a new wine. Part of the aging process is to allow these to break down over time.
What happens if you give the fermenting yeast a smaller dose? The spike in activity is smaller, the temperature will remain lower, and less fusel alcohols will be produced leading to a faster turnaround time. The yeast will also make better use of their resources. Instead of consuming the nutrients as quickly as possible, they have access to smaller amounts over the course of several days.
The rate of fermentation is smoothed out. The one large spike in speed and temperature is reduced to several smaller spikes.
This graph is a crude representation of this effect. It is strictly illustrative, not based upon real world data nor real world scale.
See our wiki article on current best-practice nutrient schedules. Context below reference the TBE schedule.
There are nearly as many schedules as there are meadmakers; however, there are limitations that dictate certain aspects of it. Most importantly, the mead maker must take into consideration which nutrients they intend to use.
Diammonium Phosphate is a conundrum. It is the densest and cheapest source of YAN and yeast strongly prefer it over organic sources. It's too easy to metabolize and causes the largest fermentation spikes. As a result, you may not wish to expose your yeast to it right away. On the other hand, it's completely ineffective in environments over 9% ABV, so it cannot always be used to supplement a sluggish or stuck must late in the process. Most common yeast nutrients contain DAP to some degree.
Organic sources of nitrogen, like those found in fruit or Fermaid O, are usable throughout fermentation and foster robust yeast health but are more expensive. They are best used very early during fermentation and again very late if necessary. They can also be used exclusively.
As long as you work within the limitations of your nutrient sources, the number of additions and specifics of your schedule come down to personal experience and preference.
In the example below, purely organic nitrogen sources are used first. Sources containing inorganic nitrogen are mixed together and added in later additions.
See Nutrient Schedules for several additional specific schedules.
The following is an example and is intended for a 5 gallon batch of a normal strength (≤14%) must fermented with a low nutrient requirement yeast, like Narbonne.
Nutrients:
12.5 g GoFerm PE (20-40 ppm YAN)
8.5 g Fermaid O (70 ppm YAN†)
8.5 g Fermaid K (45 ppm YAN)
5.5 g Diammonium Phosphate (60 ppm YAN)
Rehydrate the yeast in GoFerm PE.
After the lag phase, at the first signs of fermentation, add all of the Fermaid O.
24 hours later, add 1.83 g DAP and 2.83 g Fermaid K.
24 hours later, add 1.83 g DAP and 2.83 g Fermaid K.
24 hours later, add 1.83 g DAP and 2.83 g Fermaid K.
† This physically adds less than 20 ppm, but Fermaid O is worth 3-4x its true weight in nitrogen content for the purpose of calculating YAN. See Advanced Nutrients in Meadmaking for more information.
Advanced Nutrients in Meadmaking - a white paper by /u/Balathustrius
MeadMakr BatchBuildr - this tool will calculate the optimal amount of nutrients to add for your specific recipe