procedure TForm1.Button1Click(Sender: TObject);
var
  I: Integer;
  Columns: string;
begin
  Columns := '';
  for I := 0 to TreeView1.SelectionCount - 1 do
    Columns := Columns + TreeView1.Selections[I].Text + ', ';
  Delete(Columns, Length(Columns) - 1, 2);
  // Columns string now contains comma separated list of selected node texts
end;