This is the 6164th most frequent Russian word.
тайные
"Тайные" means "secret" or "hidden."
Here, 'тайными' is used to describe the characteristic of the notes being secret or confidential.
Они обменивались тайными записками в течение всего курса.
They exchanged secret notes throughout the course.
In this sentence, 'тайные' modifies the word 'встречи,' implying that the meetings were held in secret, away from public knowledge.
Тайные встречи проходили в укромных местах.
Secret meetings took place in secluded spots.
Here, 'тайными' describes 'желаниями,' indicating that her desires were private or hidden from others.