怎么解释这个代码
Private Sub Command1_Click()If S1.Visible = True Then
S2.Visible = True
S1.Visible = False
ElseIf S2.Visible = True Then
S3.Visible = True
S1.Visible = False
S2.Visible = False
Else
S1.Visible = True
S2.Visible = False
S3.Visible = False
End If