Can someone please explain Layer 2 Adjacency?
Put simply, a Layer 2 adjacency between two network devices simply means that these two devices are on the same network segment, the same broadcast domain, the same subnet. It means they can communicate without the need for a gateway requiring layer 3 functionalities such as routing. Two PCs connected to the same switch, and on the same VLAN are considered to have Layer 2 adjacency.
In a stricter sense, it can also mean that a data unit sent from one host can directly reach its destination without traveling through a device that would change the packet. Some would define the addition of a VLAN tag on a trunk as a change that would disqualify a communication from being Layer 2 adjacent while others would not. It depends on the requirements of the specific topology/implementation.
I hope this has been helpful!