The World Seed feature enables the display of the random seed used to generate a Terraria world, and allows the user to input a custom seed manually when generating a new world. This feature is experimental, and requires that
"UseExperimentalFeatures" is set to "true" in the config.json settings.
Worlds generated with Terraria 1.3.4 and later will always have a seed regardless of settings; The experimental feature setting merely allows the user to see, copy, and input them. If a seed is not entered manually during world generation, a seed is automatically generated randomly. Worlds generated prior to 1.3.4 will never have a viewable seed: while they were generated with a random seed, that information is simply lost as it was not stored in the world file.
When Seeds are enabled, worlds with a seed display a small seed icon in the world list, allowing the user to see that world's seed and copy it to the clipboard. A copied seed can then be input when generating a new world, which results in an identical generated world, provided it is generated with identical parameters (same size, same choice between Corruption and Crimson, and same choice between Normal and Expert).