This is the 3777th most frequent Turkish word.
çekirdek
"Çekirdek" means "seed" or "core" in English.
Here, 'çekirdek' refers to the seed of a fruit, in this case, a watermelon.
Karpuz çekirdeği çok serttir.
Watermelon seeds are very hard.
In this sentence, 'çekirdek' metaphorically means 'core' or 'nucleus' as in the central part of the family.
Ailenin çekirdeği anne ve babadır.
The core of the family is the mother and father.
Here, 'çekirdek' is used technically to mean 'core' as in a computer processor core.