PLEASE check It on code // I Can't Understand This Code //

Question

using System;
using System.Data;
using System.Windows.Forms;
using System.Data.SqlClient;
namespace InsertUpdateDeleteDemo
{
public partial class frmMain : Form
{
SqlConnection con= new SqlConnection("Data Source=.;Initial Catalog=Sample;Integrated Security=true;");
SqlCommand cmd;
SqlDataAdapter adapt;
//ID variable used in Updating and Deleting Record
int ID = 0;
public frmMain()
{
   //I Can't Understand This Code //

InitializeComponent();
DisplayData();

   //end Of I Can't Understand This Code //
}
//Insert Data
private void btn_Insert_Click(object sender, EventArgs e)
{
if (txt_Name.Text != "" && txt_State.Text != "")
{
cmd = new SqlCommand("insert into tbl_Record(Name,State) values(@name,@state)", con);
con.Open();
cmd.Parameters.AddWithValue("@name", txt_Name.Text);
cmd.Parameters.AddWithValue("@state", txt_State.Text);
cmd.ExecuteNonQuery();
con.Close();
MessageBox.Show("Record Inserted Successfully");
DisplayData();
ClearData();
}
else
{
MessageBox.Show("Please Provide Details!");
}
}
//Display Data in DataGridView
private void DisplayData()
{
con.Open();
DataTable dt=new DataTable();
adapt=new SqlDataAdapter("select * from tbl_Record",con);
adapt.Fill(dt);
dataGridView1.DataSource = dt;
con.Close();
}
//Clear Data
private void ClearData()
{
txt_Name.Text = "";
txt_State.Text = "";
ID = 0;
}
//dataGridView1 RowHeaderMouseClick Event
private void dataGridView1_RowHeaderMouseClick(object sender, DataGridViewCellMouseEventArgs e)
{
ID = Convert.ToInt32(dataGridView1.Rows[e.RowIndex].Cells[0].Value.ToString());
txt_Name.Text = dataGridView1.Rows[e.RowIndex].Cells[1].Value.ToString();
txt_State.Text = dataGridView1.Rows[e.RowIndex].Cells[2].Value.ToString();
}
//Update Record
private void btn_Update_Click(object sender, EventArgs e)
{
if (txt_Name.Text != "" && txt_State.Text != "")
{
cmd = new SqlCommand("update tbl_Record set Name=@name,State=@state where ID=@id", con);
con.Open();
cmd.Parameters.AddWithValue("@id", ID);
cmd.Parameters.AddWithValue("@name", txt_Name.Text);
cmd.Parameters.AddWithValue("@state", txt_State.Text);
cmd.ExecuteNonQuery();
MessageBox.Show("Record Updated Successfully");
con.Close();
DisplayData();
ClearData();
}
else
{
MessageBox.Show("Please Select Record to Update");
}
}
//Delete Record
private void btn_Delete_Click(object sender, EventArgs e)
{
if(ID!=0)
{
cmd = new SqlCommand("delete tbl_Record where ID=@id",con);
con.Open();
cmd.Parameters.AddWithValue("@id",ID);
cmd.ExecuteNonQuery();
con.Close();
MessageBox.Show("Record Deleted Successfully!");
DisplayData();
ClearData();
}
else
{
MessageBox.Show("Please Select Record to Delete");
}
}
}
}