This is the 816th most frequent Czech word.
docela
The Czech word "docela" means "quite," "fairly," or "completely," depending on context.
In this sentence, "docela" is used to express a moderate degree of the adjective "zajímavý" (interesting), akin to "quite" in English.
Tento film byl docela zajímavý.
This movie was quite interesting.
Here, "docela" conveys a moderate level of liking, showing that the speaker feels positively about the song but not extremely so.
Docela se mi líbí ta nová píseň.
I quite like that new song.
In this case, "docela" adds a nuance of moderation to the descriptor "náročný" (challenging), indicating that while the day was tiring, it wasn't excessively so.