This is the 275th most frequent Russian word.
самый
The Russian word "самый" means "the most" or "very (emphasizing superlative degree)."
Here, 'самый' is used to denote the superlative degree, indicating that the exam being referred to is the most difficult among a group.
Этот экзамен был самый сложный.
This exam was the most difficult.
In this sentence, 'самый' is employed to signify the superlative form of 'короткий' (short), emphasizing that the path chosen was the shortest of all options.
Она выбрала самый короткий путь.
She chose the shortest path.
The word 'самая' (feminine form of 'самый') is used here to highlight the superlative quality of the book, indicating it is the most interesting among the books the speaker has read.