Problem in duplicate Values in C#

Hello friends,
Still i m not understanding whats going on this code..... please tell me in easy way if you have any application or demo than let me know or try to send me i really need ur help...

thanks in Advance

after enetering the username, you can check this for the existance of the username in the database..(can have a button to check availability and in button click event, you can write this)

 string connString = "SERVER=" + server + ";" +
  "DATABASE=mydatabase;" +
  "UID=testuser;" +
  "PASSWORD=testpassword;";
 MySqlConnection cnMySQL = new MySqlConnection(connString);
 MySqlCommand cmdMySQL = cnMySQL.CreateCommand();   
 MySqlDataReader reader;
 cmdMySQL.CommandText = "select * from mycustomers where username = '" + txtUser.Text + "'";
 cnMySQL.Open();
 reader = cmdMySQL.ExecuteReader();
 if (reader.Read())
 {
  //user exists and perform a validation message here
 }
 else
 {
  //user does not exist
 }
 cnMySQL.Close();