Подскажите, что означает - useradd example -b /home/ -m -U -s /bin/false

Как вы собираетесь работать с линуксятиной если не можете найти элементарных вещей - синтаксис консольных базовых команд?
-b, --base-dir BASE_DIR The path prefix for a new user's home directory. The user's name will be affixed to the end of BASE_DIR to form the new user's home directory name, if the -d option is not used when creating a new account.

-k, --skel SKEL_DIR SKEL_DIR is the skeleton directory, which contains files and directories to be copied in the user's home directory, when the home directory is created by useradd.
This option is only valid if the -m (or --create-home) option is specified.

-N, --no-user-group Do not create a group with the same name as the user, but add the user to the group specified by the -g option or by the GROUP variable in /etc/default/useradd.

The default behavior (if the -g, -N, and -U options are not specified) is defined by the USERGROUPS_ENAB variable in /etc/login.defs.

-s, --shell SHELL The name of the user's login shell. The default is to leave this field blank, which causes the system to select the default login shell specified by the SHELL variable in /etc/default/useradd, or an empty string by default.

Задаюсь же вопрослм

Есть замечательная команда man.
А также у большинства команд есть ключ --help или -h