When you access your pension savings and start drawing an income, such as with an annuity or via flexi-access drawdown, you can often take some of your savings tax free. This lump sum is normally up to 25% of your pension pot (limited to a lifetime maximum of £268,275 unless you hold protection for a higher amount) and is also known as a tax-free lump sum.
(And if you had an entitlement to more than 25% as of 5 April 2006, this might still be the case now.)