Why does the game prompt me to make squares?

The best strategy to remove Dots, especially as you progress through harder levels, is to make squares.
Making a square with four or more Dots of one color will remove all the Dots of that color from the board.
If you make a larger square completely surrounding at least one Dot, each Dot inside turns into a Bomb.
This is often the best way to get a high score! Squaring isn't a catchall for winning every level, but it is the most effective way to remove a large number of Dots at once.
Two Dots has an optional feature where the game will flash a potential square when one exists on the board. If you would like to turn this feature off, you may do so in the in-game Settings menu.