NAMEreadonly - The given names are marked readonly; the values of these names may not be changed by subsequent assignment
readonly [-aAf] [-p] [name[=word] ...]
The given names are marked readonly; the values of these names may not be changed by subsequent assignment. If the -f option is supplied, the functions corresponding to the names are so marked. The -a option restricts the variables to indexed arrays; the -A option restricts the variables to associative arrays. If both options are supplied, -A takes precedence. If no name arguments are given, or if the -p option is supplied, a list of all readonly names is printed. The other options may be used to restrict the output to a subset of the set of readonly names. The -p option causes output to be displayed in a format that may be reused as input. If a variable name is followed by =word, the value of the variable is set to word. The return status is 0 unless an invalid option is encountered, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a function.
This is extracted from the main bash man page, see there for more details.