what is the difference between ambient temperature and object temperature in MLX90614 sensor
The device is an non-contact infrared temperature measurement device. Surface temperature is the average temperature (within the optical cone of the sensor) of the surface the temperature is pointed at. The ambient temperature is the temperature of the air around the sensor itself. The ambient temperature is not something I would put a lot of stock in, as it's probably reported incidentally for diagnostic purposes, as they probably use it internally to perform some kind of compensation or as an integral part of the surface temperature calculation.

The ambient temperature shouldn't change as a function of what you point the sensor at, while the surface temperature will change if you point the sensor at a cold surface compared to a hot surface.