An uncrystallised funds pension lump sum is a way of withdrawing a pension income from your pot. You can withdraw the whole pot as a UFPLS or take several pension lump sums, leaving the rest to be reinvested. 25% of the lump sum is tax-free and the rest 75% will be taxed at your marginal rate.
To withdraw an uncrystallised funds pension lump sum requires that the pot is not designated to be withdrawn as an annuity or flexi-access drawdown.