Hi Rashmi,

You can find this information also in the documentation

https://docs.mendix.com/refguide7/starting-microflows/

This is very well explained here: 

https://stackoverflow.com/questions/24714439/synchronous-and-asynchronous-data-transmission-between-client-and-server

and here:

https://www.predicagroup.com/blog/synchronous-vs-asynchronous-communication/