What it actually is
France's version of PAYE (UK) or federal income tax withholding (US). Since January 2019, income tax is no longer collected through an annual lump-sum payment in the year after earning — it is deducted monthly, directly at the source of the income.
For salaried employees, your employer withholds the tax from each payslip and remits it to the tax authorities. For pensions, the pension fund does the same. For self-employed workers, foreign-sourced income, rental income and other non-withholdable revenue, the tax authority debits monthly or quarterly « acomptes » (advance payments) directly from your bank account.
The withholding rate is personalised, determined by the tax administration based on your most recent annual income tax return. You can choose between three rates: the « taux personnalisé » (default, based on household income), the « taux individualisé » (different rates for each spouse within a couple, useful when incomes differ widely), or the « taux neutre » (anonymous default rate, applied when you don't want your employer to know your household tax situation).
Crucially, prélèvement à la source does not replace the annual tax return. You still file a declaration in May/June of each year, declaring all income earned in the previous year. The withholding is reconciled against your final tax liability — any overpayment is refunded, any underpayment is collected later.
Why it matters for you
If you receive income from outside France (UK pension, US dividends, rental income abroad), it is not automatically withheld. You will receive it gross, and you must declare it on your annual French return. The tax authority will then either set up acomptes to collect the tax monthly, or settle it in a single payment after the return is processed. Many anglophone residents are caught out the first year by an unexpectedly large tax bill — plan for it.