There are several boosts in the game: quick assist, master hint, notes master and focus.

Quick assist places a correct number in a random empty cell.

Master hint automatically places correct number in a chosen empty cell.

Notes master fills all the empty cells with the correct notes. The best way to use them is at the beginning of a level.

Focus highlights all the empty cells which the chosen number can be placed in.

❗There are also so-called "pre-boosters" which allow to speed up the solving process: 

Block notes pre-booster selects a random block, row or column and automatically fills all the empty cells of that area with the correct notes. 

Lucky cell selects a random empty cell and fills it with the correct notes.

Shield actually provides three shields. Each one protects one life, which can be lost due to incorrect move. Once all the shields are used, lives start to waste. 

To get more boosts one has to:

- complete a daily sudoku;
OR
- watch an ad: when you are running out of a boost, tap its icon and watch an ad to get a boost;
OR
- purchase boosts for coins and gems.