Telepathy is a status granted by potions of telepathy and telepathy charms. While it is active, every monster on the level will become visible on the map. They also appear on the sidebar, allowing you to see their status and health. Additionally the squares around creatures you have a telepathic link with will appear, and be added to your map as if you had seen them yourself. This can reveal items, and give you the location of hidden doors when a creature goes through them.
Monsters which are invisible, submerged or inside statues or walls will be marked with the symbol "x" for smaller creatures (such as eels) and "X" for larger creatures (such as Bog Monsters) but will not appear on the sidebar.