This is the 72nd most frequent Italian word.
detto
"Detto" means "said" or "tale" (past participle of "dire") and can also mean "saying" or "proverb."
In this sentence, 'detto' is the participle form (past tense) used here as part of the verb 'avere detto' (have said).
Hai detto una cosa interessante.
You said something interesting.
Here, 'detto' is a noun meaning 'saying' or 'proverb'.
Il detto 'Chi dorme non piglia pesci' è un proverbio italiano.
The saying 'Who sleeps doesn’t catch fish' is an Italian proverb.
In this context, 'detto' is a participle form used referentially to mean 'called' or 'named'.