timestamps tell when the data were acquired, such as date and time.  you can compare this to the time-in/out of an employee in a company.

if you want your system to be synchronized in terms of sensor readings, then the timestamps must be the same.  but in reality, this will not happen not unless your system can simultaneously read two sensors.

for me, synchronization happens when your sensor readings have almost the same timestamps, i.e. in your sensor acquisition code, the reading of the sensors must be taken sequentially.  I am assuming here a polling strategy.  an interrupt-driven acquisition works differently.