A playground was a designated area where children could play.
In her early days at Coal Hill School, Susan Foreman was flummoxed by a playground, noting that they didn't have any where she came from. (AUDIO: An Unearthly Woman)
The Twelfth Doctor observed that, during the Monk invasion of Earth in 2017, the Monks had put up statues of themselves in every playground. (TV: The Lie of the Land)