Explain The difference between the functionality 'Session.Add' vrs 'Session'

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C#

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();

}
}


Please clarify the role of Session.Add("last_name", grdStudent.SelectedRow.Cells[1].Text);

versus Session["last_name"].ToString();


Thanks