Synchronous learning refers to lessons where all the students participate at the same time, so live sessions delivered either face to face in a classroom, or live online, e.g. through Teams or Zoom.
See your guide on delivering synchronous training: Library Skills: synchronous (live and face to face) training guide.
Asynchronous lessons allow the student to use learning materials at the time of their choice, so includes recorded and self-paced material.
This guide, together with the Online Library Skills Developers' Toolkit, explains principles of asynchronous training design and delivery and use of tools for asynchronous delivery.