Although the meaning of this parameter is undefined in wxSizer, it is used in wxBoxSizer to indicate if a child of a sizer can change its size in the main orientation of the wxBoxSizer - where 0 stands for not changeable and a value of more than zero is interpreted relative to the value of other children of the same wxBoxSizer. For example, you might have a horizontal wxBoxSizer with three children, two of which are supposed to change their size with the sizer. Then the two stretchable windows would get a value of 1 each to make them grow and shrink equally with the sizer's horizontal dimension.

[size=2]wxBoxSizer* staticTextLine(wxWindow* parent, wxString label)
{
    wxBoxSizer* sizer = new wxBoxSizer(wxHORIZONTAL);
    sizer->Add(new wxStaticText(parent, -1, label), 0);
    sizer->Add(new wxStaticLine(parent, -1), 1, wxEXPAND | wxALL, 5);
    return sizer;
}

//...

leftSizer->Add(staticTextLine(this, _("Description")), 1, wxEXPAND);
[/size]