Annual leave is the British and Commonwealth term for paid holiday entitlement: 5.6 weeks statutory in the UK, four weeks in Australia and New Zealand, typically accrued through the year and requested through an approval flow. Americans call the same thing vacation or PTO, without the statutory floor.
Operationally, annual leave is a coordination input as much as a benefit: who is off shapes desk demand, anchor days and cover. That is why leave management works best sitting next to the schedule and the workplace, rather than in a separate system nobody checks before booking Tuesday.