关于一个过程我看不懂，求解

Private Sub Command1_Click()
Dim a(1 To 26) As Integer
For i = 1 To Len(Text1.Text)
If Asc(UCase(Mid(Text1.Text, i, 1))) >= 65 And Asc(UCase(Mid(Text1.Text, i, 1))) <= 90 Then
a(Asc(UCase(Mid(Text1.Text, i, 1))) - 64) = a(Asc(UCase(Mid(Text1.Text, i, 1))) - 64) + 1
End If
Next i
For X = 1 To 26
Print "字母" & Chr(64 + X); a(X)
Next X
End Sub