Open availability means a person can be scheduled across the days and shifts specified by the employer.
A shop might need cover for an opening shift, late trading or a busy weekend. Stating availability tells the manager when a candidate could work. The eventual rota tells them which shifts they have been given. A vacancy can describe both, for example by naming an expected number of weekly hours and asking for flexibility about when those hours fall. Read the days and time windows carefully. Open availability for a particular part of the week can be narrower than availability throughout the week.
In job listings
Tory Burch’s Bicester cashier vacancy asked for open availability and listed overnight, evening, holiday and weekend shifts. Those examples tell a candidate which parts of the rota they would need to be able to cover. They do not tell them how many of any one shift they would receive.