This is the 767th most frequent Estonian word.
äärmiselt
Extremely.
The word "äärmiselt" is used here as an adverb to amplify the adjective "keeruline" (difficult), indicating an extreme degree of difficulty.
See ülesanne on äärmiselt keeruline.
This task is extremely difficult.
Here, "äärmiselt" emphasizes the adjective "halvad" (bad), indicating the intensity of the bad weather conditions.
Ilmaolud on täna äärmiselt halvad.
The weather conditions are extremely bad today.
In this sentence, "äärmiselt" amplifies the adjective "tähelepanelik" (attentive), showing a high level of attentiveness.