What does it mean by delimeter

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SQL

QUOTENAME Returns a Unicode string with the delimiters added to make the input string a valid SQL Server delimited identifier.

In Microsoft website the following example was given

SELECT QUOTENAME('abc[]def');

Here is the result set.

[abc[]]def]

Can any one tell me what does it mean by delimeter Please

here is the link to that page..
http://msdn.microsoft.com/en-IN/library/ms176114.aspx[^]