This is the 87th most frequent Hebrew word.
הכי
"The most" or "the -est" (used to indicate superlative).
In this sentence, "הכי" is used as a superlative marker, preceding "מעניין" to denote 'most interesting'.
הספר הזה הכי מעניין שקראתי.
This book is the most interesting I have read.
Here, "הכי" is used to express a preference among a group, modifying the verb phrase with 'the most'.
אני אוהב את הסרט הזה הכי מכולם.
I like this movie the most out of all.
In this instance, "הכי" is used to intensify the effort being described, indicating 'the best' manner of trying.