When the player first enters a level, he gets a "level feeling" notification such as "what a boring place" or "you feel this is a superbe level" - depending on the amount of treasure and out of depth monsters on that level. This provides the player with information about what to expect on this level.
In DoomRL for example this feature can be a great help, feelings provide warning for dangerous levers (traps), and tell you if there are stairs to special levels. The different 'band versions provide hints about the monsters and treasure in the level. Even Nethack and SLASH'EM have status messages when certain levels are entered.
Roguelikes could either provide the level feeling when entering a new level, or during a level when a character had time to study the level. Some roguelikes have features related to "level feelings", such as the sound effects that are generated by vaults and shops in nethack.
Problems for developers
There are also some problems to consider with level feelings. In Angband the level feeling can be abused by the process called "level scumming", in which a player moves up and down stairs until the level feeling shows a desirable notification. This is a issue in roguelikes with level feelings and non persistent levels.
Some 'band variants have implemented different methods to prevent "level scumming". Normal Angband only shows a level feeling if a player has spend at least 100 turns on a previous level. But this is not considered to be effective, as a hundred turns is too short. Other variants have level feelings that are updated when a player spends more time on a level.