Litre is a unit of measurement for a volume, usually for a liquid. Packages of fruit juices or milk usually contain one litre. Sometimes it is also half a litre or a quarter of a litre.
The unit of measurement litre has to do with metre and centimetre: a cube with an edge length of 10 centimetres has exactly the content of one litre. It stays that way even if you pour the contents around, for example into a jug. One litre of water weighs one kilogramme at room temperature.