Synchronous communication typically means something that demands your attention as soon as something is sent. Asynchronous communication is something that typically handled whenever you're ready for it. To put it in programming terms, synchronous communication "blocks" until you deal with it, where as asynchronous doesn't require you to deal with it until you're ready to.
https://www.worldwidelearn.com/education-advisor/questions/s...
https://status.net/articles/synchronous-vs-asynchronous-comm...