I would imagine it means that the leaseholder is looking to sell the freehold of this flat to the new purchaser. I was the freeholder on a block of two flats and we had to be very clear in all our legal documents what our responsibilities were - we shared costs of work on drains and roof, for example, but external windows and doors were the responsibility of each flat owner.
That's my understanding, but you really need to ask the estate agent for clarification.