No_command can be one of two things: It can be an attribute flag or an object flag. In both cases, what it does is very similar: It disables checking for commands. On an attribute it will just stop checking that particular attribute for commands. When set on an object, all of the object's attributes will be skipped when checking for commands.
Most MUSHes will set this flag on newly created objects, since it cuts down on the amount of work the server does when trying to match a command (particularly for a 'huh'), so be sure to @set object=!no_command before trying to use any commands on it. In addition, many standard attributes (like describe, sex, and most attributes that have 'o-' versions) will have this flag set by default, since they normally shouldn't contain commands anyway.
On some codebases, the attribute flag may also prevent the checking of listen patterns. Also, some codebases will, depending on whether a player or softcode sets an attribute, automatically set this flag.