This is the 685th most frequent Bulgarian word.
най
The Bulgarian word "най" means "most" or "the most" in English.
In this sentence, "най" is used as a prefix to form the superlative degree of the adjective "добър" (good), creating "най-добрият" (the best)
Това е най-добрият филм, който съм гледал.
This is the best movie I have seen.
Here, "най" modifies the adverb "вероятно" (likely) to indicate the highest degree, making "най-вероятно" (most likely).
Той е най-вероятно в офиса си в момента.
He is most likely in his office right now.
In this context, "най" is again used as a prefix with an adjective, "добра" (good), to create the superlative form "най-добрата" (the best).