Layer2 refers back to the OSI model for networking.
Layer2 is would be devices like switches and access points. Layer3 are things like routers. Two devices on the same subnet/layer2 network can "talk" to each other directly without using a router. What this means practically for you is you need to invest in switches and access points that will actively prevent hosts from seeing each other on the same layer2 network. Client isolation on access points, VLANs or port isolation on switches.