This is the 3205th most frequent Dutch word.
geheime
"Geheime" means "secret" in English.
Here, 'geheime' is an adjective that describes the noun 'brief' (letter), indicating that the letter is secret or confidential.
Hij heeft een geheime brief ontvangen.
He has received a secret letter.
In this sentence, 'geheime' qualifies the noun 'operatie' (operation), stressing that the operation is not public or disclosed.
De organisatie werkt aan een geheime operatie.
The organization is working on a secret operation.
This use of 'geheime' modifies the noun 'locatie' (location), suggesting that the location is concealed or not revealed to many.