When you change a global item, the change is made on every page that uses the global item. For example, if you add your store logo to the Global Header & Footer, that logo will show up on almost every screen in your on-line store.
When you edit a local item, it only affects one page at a time. For example, both ABUS (About Us) and BASK (Basket) have a "local" header and footer item. If you add your store logo to the Header & Footer item on page ABUS, your logo doesn't show up on the Basket page in your on-line store.