Event line is a feature that allows you to gain additional rewards for participating in the in-game events. Rewards are divided into 2 sections - Premium (upper one) and Standard (lower one). By default, Standard rewards are available to get for all players.
Rewards can be claimed by obtaining a given number of Stars. Stars can be earned by completing event thresholds. You can check which event thresholds award Stars by entering Rewards tab in the event screen - mentioned thresholds will have Premium & Standard icon next to the threshold number.