Tomato ketchup, catsup, or just tomato sauce, is a thick, red sauce made mostly from tomatoes, sugar and vinegar. Ketchup is used on hotdogs, chips and on any other meal you think will taste nice with some ketchup on it.
A kettle is a container that is used to boil water in. They are designed to be used over a stove or fire, although the more recent electric kettle simply plugs into a power point. Kettles have a spout to pour from and a handle to hold it, very similar to a jug.
A keyboard is a device made up of a special pattern of black and white keys that is used to play music. Keyboards are found on lots of musical instruments, like accordions and pianos, as well as a range of electronic instruments like synthesizers.
A keyboard is usually found attached to a computer as a way to type words, numbers and commands with your fingers. Keyboards are also found on typewriters.
A keystone is the stone at the top center of an arch. It holds all the other stones in place, which is why it is called the keystone. Keystones are often decorated quite elaborately.
Something is khaki if it is a dull greenish brown. It is frequently used as a color for uniforms, particularly in the military.
You kick something when you hit it with your feet. A lot of sports are played by kicking a ball.