Must is the unfermented juice (for cider) or honey-water mixture (for mead) before yeast is pitched — the direct cider and mead equivalent of what wort is to beer, and the point every gravity reading, nutrient addition, and yeast-strain decision gets made from before fermentation begins.
Why must starts from a genuinely worse nutritional position than wort
Malted grain wort carries a reasonable nitrogen and micronutrient supply built into the malting process itself; apple juice and honey-water both start from a considerably poorer nutritional baseline for yeast, which is exactly why yeast nutrient additions matter so much more consistently in cider and mead making than in most beer brewing — skipping nutrient on a beer wort rarely causes a real problem, skipping it on must very often does.
No boiling, a genuinely different sanitation posture
Unlike wort, must is generally not boiled before fermentation — boiling honey or apple juice drives off delicate aromatics without providing an equivalent benefit the way boiling wort does for hop utilization and break formation, so equipment sanitation and, for cider, fruit-handling hygiene carry more of the contamination-prevention burden than they do in a beer process with a built-in boil step. That's a real, structural difference in how the whole process gets planned, not just a minor procedural detail.