What does IsDBNull(int) method do?

IsDBNull(int) method checks if a specific index you are reading from the database is null.

For example if your query was:

select 1,null;

IsDBNull(0) would have returned false (since the first field is 1) while IsDBNull(1) would have returned true (since it returns a NULL).

Note: It's also best practice to use this as a safety check before reading values from nullable columns.

Answering the question as asked...

I'm assuming you mean a MySQL version of SqlDataReader.IsDBNull(Int32) (IsDbNull(int) docs). It does what those docs say.

If you are reading a record from a data reader (something that implements IDataReader) you can test if a particular field (/column) value is NULL by using 'IsDbNull(zeroIndexedColumnNumber)`

It's not much use for checking "that [the] username and password [are] in the table in the database". For that, use the suggestion from @TimSchemlter:

SELECT 1 FROM TblUser WHERE Username=@username AND Password=@password. 

Then use MySqlCommand.ExecuteReader and check it's HasRows property