Can I still open something I have hidden?
Yes — search finds it, a saved link opens it, and any button elsewhere in the product that points at it still works. Unhide it from the same Arrange screen whenever you like.
From the guide Arranging Your Tools & Sections.
Hiding is not a permission
Hiding an entry removes it from that list and does nothing else. The page still exists, search still finds it, a link still opens it, and anybody else on the account is unaffected. If you need to actually stop someone reaching something, that is a role and permission question, not a layout one — see Roles & Accounts.
Read the whole guide: Arranging Your Tools & Sections