Timeless Trials is a weekly challenge that provides a series of gimmick maps that provides substantial rewards when each tier is completed. These gimmicks range from simply buffing certain unit types to having enemies be immune to all damage but Fixed Damage. While it can be unlocked early, to fully complete Timeless Trials one is expected to be at max level to take on the SS tier.
This game mode can be considered one of the "end-game grinds" as Timeless Trials not only provides substantial rewards, but it is also one of the few places where players can obtain SSR Accessories.
Do not neglect Timeless Trials! Progress far enough into the Trials and the player will be able to "sweep" forward to 2 tiers below the highest completed trial. In other words, you don't have to restart from the very bottom every time TT resets as you can sweep ahead to content more suitable to your level.