The optimize world screen is a game menu that is used to upgrade the level data of a world to the latest format. If the player optimizes a world, it is no longer compatible with previous versions of the game. Also, player scoreboard from datapack( at least), player advancements is erased.

Options

Erase cached data

• Cached data for the world is erased. This includes light levels and biomes, among other things.^{[more information needed]}

Create backup and load

• Creates an unoptimized backup of the world, and then proceeds with optimizing the original one.

I know what I'm doing!

• Optimizes the world without creating a backup.

History

Java Edition
16 July, 2018					Dinnerbone mentions possible addition of optimize world screen and shows early version of it.
1.13		pre10			Added optimize world screen.
		release			Added information about upgraded, skipped and total number of chunks.
1.20		Pre-release 1			Changed background texture.