This is the 5435th most frequent Turkish word.
köstebek
"Köstebek" means "mole" (a small burrowing mammal).
Here, 'köstebek' refers to the animal, mole, which is commonly found in gardens.
Bahçedeki köstebek tüm bitkileri mahvetmiş.
The mole in the garden has ruined all the plants.
In this sentence, 'köstebek' metaphorically represents a spy or an informer hidden within an organization.
Filmdeki ajan, düşman tarafta köstebek olarak çalışıyordu.
The agent in the movie was working as a mole on the enemy side.
Here, 'köstebek' is part of the name of a specific cake, the Mole Cake, known for its resemblance to molehills.