Displays a tiny red number in the bottom right corner while playing.
This is your ping (amount of ms delay with the server)
Anything below 100 should be good.

To keep the frame rate stable, it is automatically locked to 144, 60, 30, 20 or 10 fps,
depending on how fast your computer is. If you check this box it doesn't lock the framerate.
This causes a higher framerate but it feels more like the game is stuttering.

To make the game feel less laggy, the place where your player is drawn is not its actual position.
If you check this checkbox the game will draw a second dot on the position where the server thinks you actually are.

Useful for tracking the player position when making youtube videos.

When you're running too far ahead according to the server, it starts
slowing you down to make up for it. This makes sure that your land
gets filled once you reach it, instead of a couple of blocks later.
The downside is that your player is slower compared to the other players.
To prevent this, check this box. But be warned: Your blocks will be filled
with a short delay and players are able to kill you in that short time.

Hides the name above players.

In case your fps is too low. Warning! Makes the game ugly. (This is subjective)

Spectators are hidden by default. If you check this checkbox you will be able to see them.

This increases the lag,
there's absolutely no reason why you would
want to enable this unless you're debugging stuff.
This is also makes things very unstable so you might want to avoid using it.
Set this to 0 to disable it.

The version of the protocol that is used when communicating with the server. Setting this to a lower value disables some features and bug fixes. Leave empty to use the latest protocol version.