A simple command is the kind of command encountered most often. It’s just a sequence of words separated by blanks, terminated by one of the shell’s control operators (see Definitions). The first word generally specifies a command to be executed, with the rest of the words being that command’s arguments.
The return status (see Exit Status) of a simple command is its exit status as provided by the POSIX 1003.1 waitpid function, or 128+n if the command was terminated by signal n.