1 kg is how much 1 litre of water weighs. The volume of water is measured in litres, while the weight of solids is usually measured in kilogrammes. Containers are often used to store water, and the amount of water a container can hold is always given in litres.
One litre is the same as one thousand millilitres. One millilitre is the same as one thousand cubic centimetres. So, 1 millilitre is the same as 1 centimetre cubed. One gramme is also equal to one centimetre cubed. So, 1 gramme is equal to 1 millilitre or 1 cubic centimetre. This means that 1,000 millilitres of water equals 1,000 grammes. So, 1 kilogramme is the exact weight of 1 litre of water.