This function is opposite to UPPER function and Lower() is used to convert any upper case characters to lower case e.g. A, B, C, D will be a, b, c, d. The second optional parameter require name of culture like fr-FR, en-US, zh-CN and uses that culture information to do the conversion.

SELECT Lower('SQLDatabase.Net') /* returns : sqldatabase.net */;

SELECT Lower('SQLDatabase.Net', 'en-GB') /* returns : sqldatabase.net */;

The both will produce the same result of sqldatabase.net since language is English en-US as default in en-GB for UK.