Pro-rata is a Latin term which literally means 'in proportion'. When a salary is advertised as pro-rata, it basically means that the posted salary is the amount you would get if you worked full-time, all year round. Your actual salary would be proportionate, depending on the hours you work.
For example if a job was advertised as �12,000 pa pro-rata, but only working ten months of the year, then the actual salary you'd receive would be �10,000 pa. This would be equally split throughout the year, so you'd still be paid, even on months you weren't working.