In addition to the main game display terminal, which is always visible (and can show anything from dungeon maps to helpfiles), Angband and most variants allow the player to use a number of subwindows; these are optional, additional display terminals used to keep particularly important information permanently on the screen. Subwindows were first introduced in Angband 2.8.0 by Ben Harrison.
Advantages of subwindows
Commonly used subwindows include monster list, object list, pack inventory and recent messages. The monster list, in particular, is very convenient - a common cause of death is not realizing that there was a dangerous monster nearby, and this is much less likely to happen with a monster list subwindow there to alert the player.
The object list is likewise handy, allowing the player to see at a glance if there is something interesting on the current level.
When subwindows were first introduced, the main display was strictly limited to 80x24 in size; this meant that if a monitor had room for more rows and columns, the extra space could only be productively used with a subwindow. This is still the case in many old versions and variants.
Disadvantages of subwindows
Space occupied by subwindows is space not occupied by the main display; using subwindows forces the player to make compromises with either the number of rows and columns in the main display, the font size of the main display, or both. Players with bad eyesight or small monitors may find subwindows limiting rather than helpful.
Having no subwindows also allows the player to keep an iron focus on the main display. In variants actively updated today, all information available through subwindows is usually also accessible through normal commands.
Subwindows are counted up beginning at Term-1, the main display terminal being Term-0. The locations of subwindows with a given number are not fixed, and the maximum number of subwindows allowed depends on the variant and platform.
Setting up subwindows on angband.live
Set the four values on the angband.live play page - subwindow right cols, subwindow right split, subwindow top rows and subwindow bottom rows - to non-zero values before clicking Play. These values determine, respectively, the number of columns to the right of Term-0, the number of rows for the Term-1 subwindow (the subwindow immediately to the right of Term-0), the number of rows above Term-0 and the number of rows below Term-0. Standard example settings are 40/25/0/5 in that order.
To adjust what (if anything) each subwindow displays, change your subwindow options in-game; in Vanilla Angband and most variants, this can be done by typing =w during play.
If the game crashes on launch, you may have specified subwindows that occupy too much space; try lower values or a smaller font size.
To play on angband.live with no subwindows, simply keep all four values at zero.