A UK transit visa is a temporary visa that allows travelers to pass through the United Kingdom on their way to another country. It is not a visa that allows travelers to enter the UK for tourism or business purposes.
There are a number of reasons why a traveler might need a UK transit visa. For example, a traveler might need a transit visa if they are traveling from one country to another and their flight has a layover in the UK. Or, a traveler might need a transit visa if they are traveling from one country to another and they need to change airports in the UK.