Citation is the written reference to legal authorities, precedents, reported cases, etc., in briefs or other legal documents. The purpose is to provide direct proof for a statement which the writer claims to be true, so that others can verify it.
For example, a judge might use a previous legal case to support an argument that a particular law is unconstitutional. He needs to include a citation telling the reader what case it is, because other people need to be able to check whether or not he was right. To make his case stronger, he might cite more than one previous case, while his judgement and the factual accuracy of his statement might be questioned if he failed to provide any citation at all. Citations are also used in a similar way in scientific research.
A court citation is a writ or order issued by a court commanding the person named therein to appear at the time and place named.