Some content does not belong to one page — it is used in many places. Collections are central lists where this content is managed once. Pages then use entries from a collection instead of holding their own copy.
Why collections exist — the FAQ example
Imagine the same question appears on three pages. Without a collection you would have to fix a typo three times. With the FAQ collection you change the entry once — and every page that uses it is up to date right away, including published pages.
Which collections you have
That depends on your website. Look for the sidebar items starting with Collection — for example Collection FAQ. Inside, entries are organized in groups (the tabs at the top), and you can search all entries.
One entry, all languages
A collection entry contains all languages in one form — you do not switch languages like in the page editor. A translate button in the form fills the other languages for you.
Using entries on a page
Some sections use collection entries — an FAQ section, for example, lets you pick the questions it shows from the FAQ collection.
Some collections copy instead
Not every collection keeps pages up to date automatically. Some — image collections, for example — copy their values into the page at the moment you select an entry. Changing the collection later does not change pages that already use the entry. When a collection works this way, a note at the top of its page tells you.
Deleting an entry
Be careful when deleting: an entry that pages still use disappears from those pages. Only delete an entry when you are sure no page needs it anymore.