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It means you must hold a ticket out of the UK within the specified time. You can't present yourself to the immigration checkpoint and tell the officer you will buy a ticket before leaving and you can't use this facility to stay 2 or 3 days in the UK.
If you choose widely, you could have more than 24 hours to do some sightseeing but basically, it's a concession to allow you to collect and re-check your bags or switch airports but not a way to circumvent the visitor visa requirements (and fee) if you really intend to stay a few days in the UK.
Note that you must also fulfill one of the conditions listed further down the page.