Yeast nutrient is a supplement — commonly diammonium phosphate (DAP) alone, or a more complete blended commercial product covering a wider range of micronutrients — added to cider or mead must to make up for nitrogen and micronutrient levels that apple juice and honey-water both fall genuinely short on compared to malted-grain wort.
Staged additions beat a single dose, especially for mead
Rather than dumping the full nutrient dose in at pitching, many mead makers stagger additions across the first several days of active fermentation instead — early, healthy nutrient availability matters most while yeast is establishing itself, and staged dosing avoids overwhelming the must with nutrient all at once while still supplying what a demanding, high-gravity fermentation needs as it progresses.
Skipping it is the single most common cause of a stuck fermentation
A must fermented without adequate nutrient risks stressed, incomplete fermentation — sometimes stalling well short of the target gravity, sometimes finishing but with noticeable off-flavors from stressed yeast metabolism along the way. This matters even more at the higher gravities Traditional Mead and Melomel commonly reach, where yeast is already working harder than in a standard-strength ferment and has even less margin for a nutrient shortfall, making staged nutrient dosing less of a refinement and more of a real requirement.