In the bottom right corner of the main screen you will see the Star Pass button. Clicking it will take you to the Star Pass menu. Each new Star Pass launches a new theme Season. Basically, a Season is a set of rewards, access to which opens gradually with the Episodes of the season. During each Season, you can earn Stars and fill up the progress bar to collect various rewards.

To consistently advance from Episode to Episode and receive rewards, you need to earn Stars by fighting in battles. Stars can also be found in chests as a reward for any in-game activity. As you collect Stars, your Season progress bar will fill up, and when you reach a certain milestone, you will be able to collect the reward.

These milestones are called Tiers - please don't confuse Tiers with Episodes. A Season consists of Episodes and Episodes, in turn, consist of Tiers, which are the rewards on the progress bar.

Episodes have opening timers, and if you have reached the last Tier of an Episode and collected all the rewards, you can just keep earning Stars: they will automatically be applied to the Tiers of the next Episode when it opens.

Even if you haven't reached the next Tier yet, you can get it immediately by purchasing it for bucks:

In the Tiers you will see some locked rewards and the rewards that are marked "Free". Free rewards can be obtained without any special requirements, while locked rewards are unique rewards that require Premium Episode Pass to get them. 

There are 2 Pass options to choose from: Premium Episode Pass and Premium Episode Pass Plus. Both will only unlock the rewards of the specific Episode that you purchase them for. This means that to get the rewards of Episode 1, you'll need to buy a Pass for Episode 1. Then, if you want to get Episode 2 rewards as well, you'll need to buy a Pass for Episode 2 additionally. Please also note that in order to purchase the Episode 2 Pass, you'll need to buy the Episode 1 Pass first (the same applies to Episode 3, which requires Episode 1 and 2 Passes first). 

When you choose a Pass, you can see what perks and rewards each Pass would give you. Unique Episode rewards are the rewards that you'll receive immediately after purchasing a Pass. Both Passes offer different unique rewards, so check the list to decide which Pass is best for you. As for the locked Tiers, both Passes will unlock them for you on the progress bar.

Also note that you can only buy one type of Pass for each Episode, so if you buy the regular Premium Episode Pass for Episode 1, you won't be able to buy Premium Episode Pass Plus later for the same Episode. Choose wisely!

Suppose you decide to buy a Premium Episode Pass after you have completed a few Tiers. In this case, all previously locked Episode rewards will be unlocked for you to collect as soon as you receive the Pass.

And don't worry about any uncollected rewards: once the new Season begins, the game will offer to collect any reached rewards from the previous Season that you may have missed out on for whatever reason.