The apnea-hypopnea index (AHI) is the headline number from a sleep study. It counts how many times per hour of sleep your breathing either stops completely (an apnea) or becomes shallow enough to drop your oxygen or briefly wake you (a hypopnea). Fewer than 5 events an hour is considered normal; 5–14 is mild sleep apnea, 15–29 moderate, and 30 or more severe.
The AHI is how clinical trials measured the effect of GLP-1 treatment on sleep apnea — the tirzepatide trials that led to its approval for obstructive sleep apnea reported roughly a halving of AHI — and it's the number a repeat sleep study uses to decide whether apnea has improved enough to change CPAP settings or stop. See the guides on getting a sleep study and sleep quality on a GLP-1.