This is the 7996th most frequent Russian word.
корешки
"Корешки" means "roots" (as of plants) or can colloquially refer to close friends/buddies.
In this sentence, 'корешки' refers to small roots growing on a tree, highlighting its biological or botanical meaning.
На древнем дереве росли корешки.
There were roots growing on the ancient tree.
Here, 'корешки' metaphorically represents the 'roots' or origins of words in the context of linguistics.
Мы изучали корешки русских слов в классе.
We studied the roots of Russian words in class.
In this figurative usage, 'корешки' colloquially refers to companions or friends, depicting a casual or friendly relationship.