This is the 5402nd most frequent Portuguese word.
secretos
"Secretos" means "secrets" in English.
Here, 'secretos' is used as an adjective describing 'documentos', specifying that they are 'secret' documents.
Eles guardam documentos secretos no cofre.
They keep secret documents in the vault.
In this sentence, 'segredos' is used as a noun to mean 'secrets', referring to hidden or undisclosed information.
Os segredos da juventude muitas vezes são ocultos.
The secrets of youth are often hidden.
Here, 'secreta' (a related form) is used as an adjective modifying 'operação', implying that the operation is conducted in secrecy.