Some puzzles, called Starry Puzzles, are locked. In order to unlock them, you must collect the right number of stars.
An example of a starry puzzle
During the OdySight Adventure, you receive from 1 to 3 stars each time you successfully complete a puzzle. The quicker you complete the puzzle, the more stars you will obtain.
Here, to obtain the second star, the player has to solve the puzzle in 2 minutes and 33 seconds maximum
If you reach a puzzle with a padlock on it and you do not have enough stars, you will have to redo the puzzles in which you did not obtain 3 stars. When you click on one of these levels, the game shows your best score and the maximum time allowed to obtain the next star. For example, if your best score is 1 star, the game tells you the time limit required to obtain the 2nd star. Likewise, if your best score is 2 stars, the game tells you the time limit required to obtain the 3rd star.
The number of stars is shown in the red circle in this picture
You can see the number of stars you have at the top left of your screen when you are on the map.