Lockets are a special type of Puzzle level goal that cannot be damaged by matches or Boosters.
Instead, to collect a Locket, you'll need to clear the obstacles and pieces beneath it, allowing it to drop onto the collection area, which is marked by a downward-facing arrow:
The collection area is usually at the bottom of the Puzzle board, but this may also vary by Puzzle level.
To clear the obstacles and pieces beneath the Locket, you can use different strategies, such as making matches or activating Boosters: