The brief command toggles your verbose/brief mode setting. When you enter a room in brief mode, you only get the "short" description and the exits of the room (like typing gl or glance). If you enter a room in verbose mode, you get the entire room description (like when you type look or l). Typing brief will change this setting and tell you what your current setting is.
It used to be that brief mode was helpful during dialup situations in which case the less text sent, the more responsive your experience was. However, now, we can offer more information while still maintain a level of brevity to the information imparted.
With that, there are now two levels of brief. Regular brief and full brief mode. Full brief mode, achieved by typing "brief full", will continue to only impart the short description and the exits in your current room. However, regular brief, will now show, in certain circumstances, additional information that will be of assistance in navigating the game. Examples of such things are hitches, and room effects that get added via abilities or other means, plus some directional assistance. House addresses are one of the main items under current development to bring them to the brief mode, so you will see in some areas where a house address is shown, but in others they may not. Please do not report these, this will take some time.