Session Add need clarification

grdStudent.DataSource = ds.Tables[0];
       grdStudent.DataBind();

       lblConnectionState.Text = "RESULTS RETURNED";

   }

   protected void grdStudent_SelectedIndexChanged(object sender, EventArgs e)
   {
       //GridViewRow row = new GridViewRow();

       Session.Add("last_name", grdStudent.SelectedRow.Cells[1].Text);
       Session.Add("other_names", grdStudent.SelectedRow.Cells[2].Text);
       Session.Add("marital_status", grdStudent.SelectedRow.Cells[3].Text);
     
       if (IsPostBack)
       {
           txtLastname.Text = Session["last_name"].ToString();
           txtOthernames.Text = Session["other_names"].ToString();
          ddlMaritalStatus.SelectedValue = Session["marital_status"].ToString();
       
       }
   }

<pre lang="cs">grdStudent.DataSource = ds.Tables[0];

Session.Add("last_name", grdStudent.SelectedRow.Cells[1].Text);