Synchronous learning refers to online class sessions and activities that happen in real time with an instructor, typically using Zoom or other online meeting tools.
Asynchronous learning occurs without real-time interaction through online modules, resources and learning activities prepared by the instructor, such as assignments, discussions, etc. and often organized and facilitated in a course site.
Hyflex instruction combines in person (physical class) and online synchronous (online class) students in one class session. See Hyflex teaching at the University of Minnesota.