You sense any trap within range that is within line of sight. A trap, for the purpose of this spell, includes any object or mechanism that was created to cause damage or other danger. Thus, the spell would sense the Alarm or Glyph of Warding spell or a mechanical pit trap, but it wouldn't reveal a natural weakness in the floor, an unstable ceiling, or a hidden sinkhole.
This spell reveals that a trap is present but not its location. You do learn the general nature of the danger posed by a trap you sense.