Synchronous means 'ordered communication'. When we do synchronous operation everything that comes later has to wait for the operation to finish before they can start.
Asynchronous means 'unordered communication. Asynchronous is usually used to describe communications in which data can be transmitted intermittently rather than in a steady stream.

For ex: a telephone conversation is asynchronous because both parties can talk whenever they like. In synchronous communication each party would be required to wait a specified interval before speaking.