Synchronous means that you call a web service (or function or whatever) and wait until it returns - all other code execution and user interaction is stopped until the call returns.
Asynchronous means that you do not halt all other operations while waiting for the web service call to return. Other code executes and/or the user can continue to interact with the page (or program UI).
You can even check out the details of automation testing tool with the Software testing course online.
Hope this helps!