This is the 772nd most frequent Finnish word.
paljastaa
To reveal or disclose.
Here, 'paljastaa' is used in the sense of 'revealing' or making something previously unknown, such as the truth, known to others.
Hän päätti paljastaa totuuden ystävilleen.
She decided to reveal the truth to her friends.
In this context, 'paljastaa' implies the gradual unveiling or showing of something, in this case, the petals of the rose.
Ruusu alkoi paljastaa kauniit terälehtensä auringonpaisteessa.
The rose started to reveal its beautiful petals in the sunlight.
Here, 'paljastaa' is applied to describe the act of unintentionally or subtly making known one's inner emotions or thoughts through actions.