Relics are in-game collectables.
Relics are a feature that were added in the Nook And Cranny Update (0.5.3). They come in two forms: The conventional forms of Relics (fossils, statues, etc.) or Messages inside bottles. In most biome, 3 to 5 Relics are available. The Micro Depths and the final biome do not have any Relics. The Messages inside bottles are global and can be found in any biome. Relics can only and rarely be found inside small piles of rubble, which are found exclusively in Caves.
The primary purpose of Relics is to be something for the player to collect, and their descriptions are usually very entertaining. However, there might be other uses for certain Relics. Any Relics gathered can be viewed in the "Relics" category in the second menu.
Please beware for spoilers.