The meaning of timestamps

hey. I’m a beginner in px4 developing so sorry about my simple question. In structures containing uORB topics there are fields named timestamp, timestamp_sample, ref_timestamp, etc.
I can’t uderstand the difference between them. for example “timestamp”: does it contain the time of associated topic update or it stores the absulute time independently?
what about timestamp_sample?
they both are in the vehicle_local_pos_s structure.