Starting every month a new battlepass will be unlocked. You can find this by navigating in your menu to the 'Rewards' tab, where you would collect your Loyalty Rewards.
You obtain XP by playing the game! This includes killing enemies and bosses in raids. The world bosses also chip in on your progression.
After completing each tier you will unlock a reward, as seen below, where you will be able to clicky-clicky the box and your character will collect the item.
There are 50 tiers in total to each pass and you will have the whole month to reach it. Not everyone will, but it's worth it to those that do!
Don't worry though, next month the tier 50 reward will be back for you to try again!
Good luck and hopefully you get something that kicks ass!