"5pm sharp" means "exactly 5pm". It means that there are no exceptions after 5pm. For example, "meet me at 8:00 sharp" means "meet me exactly at 8:00". Your sentence could be translated to "Weekly orders will have to be in by exactly 5pm", or "no later than 5pm". The sharp puts emphasis on the fact that there are no exceptions surrounding the specific time. Hope this helps, sorry if my answer is confusing!