Employees have their income tax deducted automatically from each salary through PAYE. People who earn income outside that system — freelance fees, business profits, certain investment income — have no employer withholding tax for them, so SARS collects it through the provisional tax system instead.
Provisional tax means estimating your taxable income for the year and paying it in advance instalments, rather than facing the full amount only after your annual assessment. This smooths cash flow for both the taxpayer and the fiscus and avoids a large year-end bill.
Whether you are a provisional taxpayer, and how much you must pay, depends on your income and circumstances against thresholds set by SARS. This is general information — confirm your own status and obligations with a registered tax practitioner or SARS.