UK Visitor Visa, also known as a Temporary Resident Visa or an UK Tourist Visa, is a legal document that must be carried in your passport to prove that you have fulfilled the requirements for temporary resident entrance to UK (either as a visitor, a student, or a worker).
Most travellers to UK are permitted to stay for up to six months. The immigration border services at the port of entry may permit you to stay for less than or more than six months. They will record the departure date in your passport once they have given you a stay period. The immigration official may also offer you a visitor record, which will include the departure deadline.