The phrase "qualifying nights" refers to the number of nights someone needs to stay at a specific place in order to meet certain requirements or qualify for certain benefits or rewards. Full definition
Each card by itself offers 2 elite qualifying stays and 5 elite qualifying nights per year. (travelcodex.com)
The targeted offers for double qualifying nights were never sent my way, and I ended up losing my Platinum status. (travelcodex.com)
The qualification to gift status is to reach 60 or 100 elite qualifying nights within the calendar year and only those who achieve that many nights are eligible. (hiltonhonors3.hilton.com)