Watershed (television)
Today, television can show a number of programmes. Usually the content of these programmes is rated, by a number of people (usually called advisory board). Some of the programmes may be suitable for children (of different ages), other content may not be suitable for children. The watershed is the time of the day where content not suitable for children may be shown. This is usually during the night (starting in the later evening, and ending in the early morning)
Content that children should not see
The content that is not suitable for children to see usually includes sex or nudity outside a functional context. People bathing naked in a lake may be ok, people doing a striptease in a night club or cabaret may not be. Other content generally not suitable includes various levels of violence or the use of illegal drugs. Sometimes strong language (full of expletives) is also seen as unfit for children.