Jetpack Joyride

Missions are side objectives for you to work towards while playing the game. They generally require you to either complete a task in a single run of the game, or complete cumulative requirements across many plays. Missions can be such things as passing a given number of red lights, going a certain distance in a vehicle or without collecting any coins, etc. At any one time, there are three missions active. You can see what they are from the Pause Menu in-game, or by clicking on "Missions" from the Main Menu.

In total, there are around 50 unique missions to complete. They do progressively get harder in most cases, but not much. Each mission is given a star rating (from 1 to 3) which is very loosely based on the difficulty. After completing the mission, you are awarded the difficulty stars which total towards leveling up. There is an achievement for completing your 10th and 40th missions. Additionally, you will then need to reach and fill all the stars for Level 15: Barry. After doing so, you will be given the option to "Cash In" your missions and reset them to complete them again, earning the "A man, my son" achievement. NOTE: You DO NOT lose anything by cashing your missions in; you just get an achievement and the ability to cycle through the missions again. 
Keep your current missions in mind and work towards completing them whenever you can. Otherwise, if you ignore them, you will have a decent grind ahead of you at the end. Some of the final missions can be a little challenging. If you are definitely stuck, you can go into the Stash>Utilities and purchase automatic mission completions (5k for 1 star mission, 10k for 2 star mission, and 15k for 3 star mission). However, this is a lot of money and with a little practice and patience, you should really not have to much trouble with any of them.

You will need to level up through the 15 ranks (listed below) by completing missions. You will have three active missions at the start of each run which provide between 1-3 stars each. If you are having trouble with a particular mission, you have the option to purchase a Complete Mission token from the Utilities menu.

Ranks:
Rank 1 – Beginner (3 Stars)
Rank 2 – Learner (3 Stars)
Rank 3 – Rookie (3 Stars)
Rank 4 – Novice (3 Stars)
Rank 5 – Amateur (3 Stars)
Rank 6 – Graduate (3 Stars)
Rank 7 – Skilled (3 Stars)
Rank 8 – Experienced (7 Stars)
Rank 9 – Professional (9 Stars)
Rank 10 – Expert (11 Stars)
Rank 11 – Hotshot (11 Stars)
Rank 12 – Wizard (11 Stars)
Rank 13 – Ninja (11 Stars)
Rank 14 – Superstar (11 Stars)
Rank 15 – Barry (13 Stars)

You will need a total of 105 Stars to complete all 15 Ranks. When you finished, the game will ask you if you want to Cash in Your Rank or Save for Later. Select Cash In and the achievement will pop.

You will not lose your gadgets, jet packs, suits, etc. It just resets your rank and continues to give missions.