Increasing your rank in Rise of the Blobs opens up new themed stages for you to play on. If you want to increase your rank, keep playing and chasing a higher score, as each match will reward you with experience which fills up the rank meter.
Rainbow fruits will occasionally spawn on the game board, and are capable of absorbing fruits of any colors. Handy if you want to keep your game going longer.
TNT Blobs are great for destroying a large number of nearby blobs, making the game area easier to work with.
If you're struggling to get much of a high score in the Timed mode, you might want to think about spending some coins on Time Bonus.
Cherry Bonus will add a bonus to your score, based on a percentage of your total score.