The Elisp manual is your friend. C-h i
, choose Elisp. Then i
and enter read-command
. As part of the description you see this:
The argument DEFAULT specifies what to return if the user enters
null input. It can be a symbol, a string or a list of strings.
If it is a string, `read-command' interns it before returning it.
If it is a list, `read-command' interns the first element of this
list. If DEFAULT is `nil', that means no default has been
specified; then if the user enters null input, the return value is
`(intern "")', that is, a symbol whose name is an empty string.
Read the full description. But this alone should help you understand. The value returned is a symbol that has an empty name.