How did the readonly word implemented?

[winterTTr]

I am really a new one to emacs.
I am using some package for emacs like ido, and i just want to know
that, how did the readonly character implement?

I mean for the thing such as, when you C-c C-f, and you'll find that
there is some thing like { xxx | xxx| ... }, and they seems not
editable.
Is this a emacs internal feature which can make a character readonly?
Or there is a lisp written function to make this thing possible, such
as "eat" all input when you on some character ?

Can anyone explain some information of this feature to me?