Braced is a state in which a character will not be able move or be moved.
As a result they have a +2 to AC and +4 WC penalty.
Any experience gained whilst braced is reduced to a 5th.

Bracing can be useful for staying on top of an enemy god's altar whilst praying if the character wishes to convert.

To toggle the braced status: Use the brace command.

This, along with fire are the most common states for new players to accidentally get confused into thinking they are stuck. It can easily be switched by using the brace command again to toggle it off.

Text of the help file brace.en inside {prefix}/share/crossfire/help folder of the server installation :

Brace:
When you enter the 'brace' command to brace your character, your character will no longer move.
It can still attack adjoining spaces.
Bracing can be useful to hold a location.

When you are braced, you lose your dex bonus and incur a 2 point ac penalty beyond that

(if you have a negative dex bonus, you may in fact come out ahead.

You also only get 20% of the normal experience for killing creatures,
and incurs a 4 point wc (to hit) penalty.