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.

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.