This is the 3218th most frequent Russian word.
непросто
"Not easy" or "difficult."
In this sentence, 'непросто' is used to describe the complexity or difficulty associated with living in a big city.
Жить в большом городе непросто.
Living in a big city is not easy.
Here, 'непросто' emphasizes the demanding nature of mastering the skill of playing the violin.
Научиться играть на скрипке — это непросто.
Learning to play the violin is not simple.
The sentence uses 'непростой' in its adjectival form to describe the surprising difficulty of the task in question.