https://tfl.gov.uk/fares/how-to-pay-and-where-to-buy-tickets...
[Edited to add: Stupid web site doesn't have working fragment markers, click "Same Station Exits" near the top... ]
In practice, as it explains elsewhere on the site, these charges are mostly to discourage attempted fare evasion, and if you're a regular user who just does this once in a while it'll delete the journeys after it decides this isn't suspicious after all, otherwise you need to talk to a human if you really didn't travel anywhere.