In Minecraft Bedrock Edition, experimental gameplay allows using new features that are not available in the regular game yet.
There are two different ways to enable experimental features on your server.
Uploading a world (recommended)
The best option to use experimental gameplay is to generate a world with experiments enabled in single-player-mode and upload it to your server.
Editing your level.dat file
If you want to use experiments in an existing world rather than generating a new one, you can enable them in your world's level.dat file, which you can access by clicking the Options button on the Worlds page.
Warning: Enabling experiments in existing worlds is not recommended and can cause problems.