The Secure Application will prevent end users from accessing the Visual Basic editor. This does not really protect your VBA code, contrary to the XLS Padlock’s VBA compiler. However, it is useful to limit possible VBA hacks (password retrieval, export of the workbook to an external file…)

Please also refer to the dedicated topic about stopping access to VBA editor.

Prevent common VBA and OLE hacks   |   Disable common Excel add-ins