It’s a Z-Wave door/window contact sensor (model ZW120, sometimes ZW120-C or ZW120-D) from Aeotec’s “Gen5” line.
Its job: to detect when a door or window is opened/closed, and report that event wirelessly to your Z-Wave gateway/hub.
Supports secure communication (Z-Wave security) and OTA (firmware upgrade) in many versions.
Uses 2 × AAA batteries.
Indoor use, and best installed with the main sensor on the moving part (door/window) and magnet on the frame. Gap less than ~2 cm recommended.
Here are some of the key specs (note: always check variant for your region as frequencies differ):
Operating temperature: 0 °C to 40 °C.
Operational humidity: 8 % to 80 %.
Battery: 2 × AAA; battery life up to ~1 year (depending on usage) according to Aeotec.
Wireless range (free field, outdoor): up to ~150 m / ~492 ft.
Gap between magnet & sensor: recommended less than ~2 cm.
It supports many Z-Wave command classes, including battery, notification, sensor_binary, security etc.