The Supplementary Retirement Scheme (SRS) is a voluntary scheme to encourage individuals to save for retirement, over and above their CPF savings. Contributions to SRS are eligible for tax relief. Investment returns are tax-free before withdrawal and only 50% of the withdrawals from SRS are taxable at retirement.
If you require more information on SRS, you may refer to the Ministry of Finance's (MOF) website and the SRS FAQs.