General.
Ticks per second: how fast the game runs. 20 is normal Minecraft speed.

Lower slows everything down (crops, mobs, day and night), higher speeds it up.
PVP.
On: players in Creative mode can't be hurt by anything except the void.

Off: they take damage like everyone else.

Worldborder.

Control.
Enabled: Only players that you add will be able to join. Use the whitelist commands.

Permissive (recommended): Anyone can join while you (the server owner) are online and get automatically whitelisted. As soon as you log off only players who you have manually added to the whitelist or have joined prior can join your server.

Disabled: Anyone can join the server at any time.
Applies 2 optimisations:
  1. In regions with a lot of TNT, particles or generally entities the number of each per block gets capped and any additional do not get sent to the client to reduce lag.
  2. A hard limit of entities/particles/tnt is applied, and if it exceeded then entities/particles starting furthest away from the player are not sent to the player.
Both of these optimisations reduce bandwidth usage (mobile data!) and improve client side performance. Nothing is deleted server side :)

Installed plugins.
All plugins.

Are you sure you want to delete your server forever?

Server status:
Red: The server is currently offline. No one can join.
Orange: The server is currently either stopping, starting or restarting. This state usually only lasts for a few seconds.
Green: The server is running and you can join instantly.
Blue: The server is currently offline, however if you try to join the server will start up quickly and you won't need to wait to start the server. The server automatically hibernates after 5 minutes without any players online