The Read-only Registers

The includes three additional registers called the "read-only" registers.

Contains the most-recently inserted text. This is updated each time Vim is in insert mode, and allows (for example) the same text to be pasted into the document multiple times.
Contains the name of the current file, which makes it useful in commands that operate on the entire file.
Contains the most recent command that was executed on the command-line. As with the other registers in this section, this can be used to repeat the same multiple times without having to type it in each time.