This is the 893rd most frequent Swedish word.
hemlig
Secret.
Here, "hemlig" is used as an adjective describing the plan that is kept undisclosed or confidential.
Han avslöjade en hemlig plan.
He revealed a secret plan.
The term "hemlig" functions as an attribute of the location, emphasizing its concealed nature.
De träffades på en hemlig plats.
They met at a secret location.
In this sentence, "hemligt" describes an activity or something undefined that is kept unknown, reflecting the inferred mysteriousness.