谁能帮我解释一下这个？（高分）

unit main; //单元

interface

uses //用到如下单元
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
FileCtrl, StdCtrls, ExtCtrls, Buttons,Jpeg, MPlayer;

type //各种控件的声明
TMainForm = class(TForm)
DriveComboBox1: TDriveComboBox;
FilterComboBox1: TFilterComboBox;
DirectoryListBox1: TDirectoryListBox;
FileListBox1: TFileListBox;
Panel1: TPanel;
MediaPlayer1: TMediaPlayer;
BitBtn1: TBitBtn;
BitBtn2: TBitBtn;
procedure FileListBox1Click(Sender: TObject);
procedure BitBtn1Click(Sender: TObject);
private //私有变量或者方法的声明
{ Private declarations }
FileExt:String[4];{存放文件扩展名}
public //公有变量或者方法的声明
{ Public declarations }
end;

var
MainForm: TMainForm; //窗体名称

implementation

uses Bigpas;

{$R *.DFM} //下面是具体的方法（函数）begin end 相当于{}
procedure TMainForm.FileListBox1Click(Sender: TObject);//选择文件播