Warframe’s Nightwave is basically the equivalent of a battle pass in games like Fortnite, Call of Duty, Rocket League, and many others where there is a periodic rotation of different challenges to complete. These rotated challenges help the player in progressing through the pass. What this does along the way is that in every newly unlocked tier, you get a new item exclusive to the pass.
Of course, the items differ in each respective game. However, in Warframe’s case, all players get access to this system at no additional fee or anything. Each challenge is an act, but the terminology is interchangeable. Each challenge category is also categorized by Daily, Weekly, and Elite, of course with the latter being the more time consuming or challenging, but with more standing gains.
The only barriers to completing the pass are either time constraints and unmet quest prerequisites. I will say this, you may want to have most major questlines done or close to completion. You may also want to have some fairly decent builds and a diverse arsenal.
Most acts shown are what is available from the Wiki as long as the new ones added in the beginning of Nightwave Intermission 3.