What it does, precisely
It records each copy. The app watches the system clipboard and saves a copy of every item you put there: text, links, images, files, rich text from a document. Each entry keeps where it came from, so you can tell the two similar-looking snippets apart later.
It gives you the list back on a shortcut. You press a key combination, a small window appears with your recent items, you pick one and it pastes into whatever you were typing in. Good ones let you type a few letters to filter, so finding an address you copied an hour ago takes about two seconds.
Some of them do more than remembering. Pasting without formatting, saving snippets you reuse, reading text out of a copied screenshot, cleaning up line breaks from a PDF. Those are extras. The list and the shortcut are the product.
It stores the history locally by default. In most Mac clipboard managers the history is a database file on your own disk, with no account and no server involved. Sync between devices exists in some of them and is normally something you switch on.
What macOS 26 Tahoe added
A useful history list shows the copied item's source app and content type on each row, and lets you search across all of it. Filtering by type matters more than it sounds, because a day of copying produces mostly text and the one image you want is buried in it.
Images are the part built-in history does not cover. A copied screenshot has to be stored outside the database and looked up by content hash, otherwise the history file grows enormous and the same image gets saved four times.
macOS 26 Tahoe added a clipboard history to Spotlight. Press ⌘Space to open Spotlight, then ⌘4, and you get your recent copied items. It keeps them for 8 hours by default, and the retention choices are 30 minutes, 8 hours or 7 days. It survives a reboot. The Clipboard switch lives in System Settings under Spotlight, so that is where to look for it.
The limits are worth knowing before you decide you are done. It handles plain text only, so copied images and files are not in there. There is no text recognition inside a copied screenshot. And it is macOS 26 only.
On macOS 15 Sequoia and anything older there is no built-in clipboard history at all. Nothing hidden in a menu, nothing to enable. If you are on Sequoia, a third-party app is the only route. There is no macOS 25, in case a search result told you otherwise.