Visual Basic.NET - ficheros en visual basic. net

hola me pueden explicar todo el codigo porque de el no entiendo nada me seria de gran ayuda gracias.
Public Class Form1
Inherits System.Windows.Forms.Form
Private Sub mnu_abrir_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles mnu_abrir.Click
OpenFileDialog1.Filter = "Archivos de texto (*.TXT)|*.TXT"
OpenFileDialog1.ShowDialog()
fich_abierto = OpenFileDialog1.FileName
If OpenFileDialog1.FileName <> "" Then
Call carga_datos(fich_abierto)
Panel1.Text = fich_abierto
mnu_abrir.Enabled = False
mnu_cerrar.Enabled = True
End If
End Sub
Private Sub mnu_Guardar_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles mnu_guardar.Click
Call guarda_fich(fich_abierto)
StatusBar1.Text = SaveFileDialog1.FileName
mnu_cerrar.Enabled = True

End Sub
Private Sub mnu_Guardacomo_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles mnu_guardar.Click
SaveFileDialog1.Filter = "Archivos de texto (*.TXT)|*.TXT"
SaveFileDialog1.ShowDialog()
If SaveFileDialog1.FileName <> "" Then
Call guarda_fich(SaveFileDialog1.FileName)
StatusBar1.Text = SaveFileDialog1.FileName
mnu_cerrar.Enabled = True
End If
End Sub

Private Sub mnu_cerrar_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles mnu_cerrar.Click

FS.Close()
ListBox1.Items.Clear()
TextBox1.Text = ""
Panel1.Text = ""
mnu_abrir.Enabled = True
End Sub

Private Sub mnu_salir_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles mnu_salir.Click
End
End Sub

Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
mnu_guardar.Enabled = True
End Sub

Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
dato_activo = ListBox1.SelectedIndex
TextBox1.Text = agenda(dato_activo)
End Sub

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
If Button1.Text = "Modificar" Then
TextBox1.Enabled = True
Button1.Text = "Guardar"
Else
agenda(dato_activo) = TextBox1.Text
Button1.Text = "Modificar"
TextBox1.Enabled = False
Call carga_list(ListBox1)
End If
End Sub

Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
Panel3.Text = Hour(Now()) & ":" & Minute(Now()) & ":" & Second(Now())
End Sub

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Panel2.Text() = Today
Panel4.Text = "AGENDA"
End Sub
End Class

Imports System.IO
Module Module1
Public agenda(30) As String
Public tot_datos As Byte
Public dato_activo As Byte
Public fich_abierto As String
Public FS As FileStream
Public f_Lee As StreamReader
Dim f_Escr As StreamWriter

Public Sub carga_datos(ByVal nombre_f As String)
Dim dato As Object
FS = New FileStream(nombre_f, FileMode.Open)
f_Lee = New StreamReader(FS)
dato = f_Lee.ReadLine()
tot_datos = 0
While dato <> Nothing
agenda(tot_datos) = dato
dato = f_Lee.ReadLine()
tot_datos += 1
End While
f_Lee.Close()
End Sub
Public Sub guarda_fich(ByVal fichero)
Dim dato As Object
Dim i As Byte = 0
FS = New FileStream(fichero, FileMode.Create)
f_Escr = New StreamWriter(FS)
For i = 0 To tot_datos
f_Escr.WriteLine(agenda(i))
Next
f_Escr.Close()
End Sub

Public Sub carga_list(ByVal listado As Object)
Dim i As Byte
listado.Items.Clear()
For i = 0 To tot_datos - 1
listado.Items.Add(agenda(i))
Next
End Sub
End Module