If you are new to The 100 , Season 1 is essential. It introduces the core concepts:

Upon their arrival on Earth, the group, known as the 100, quickly discovers that they are not alone. They encounter the Grounders, various tribes of people who survived the apocalypse and have been living on Earth. The Grounders are diverse, with different cultures, languages, and customs. The main character, Clarke, emerges as a strong leader among the 100, navigating through the complex web of alliances and rivalries between the Grounders.