There are different stages in a property transaction. Until exchange, either party is free to withdraw without any contractual penalty.
When “exchange” occurs, the parties are bound to complete on the completion date. If either withdraws, there will be financial consequences.
“Completion” is the date upon which the keys will change hands once the purchase price has been transferred.
Occasionally it is possible to exchange and complete on the same day, but it is more usual for there to be a gap of one or two weeks in between.