For the KET exam you need the kind of English that a visitor to an English-speaking country should have. You need to understand simple statements of fact, signs, notices, and instructions from people such as police officers. You will also need to be able to give basic information about yourself, and to give and receive simple messages. These messages may be written or spoken, for example on the telephone.
People who want to do the test must know how to introduce themselves and other people, and how to talk about simple things such as the weather, their sports and their hobbies. When they have not understood, or they do not understand the other person, they must know how to say this, and try to explain, or ask the other person to explain again.