The Open Systems Interconnection (OSI) model has seven layers that define network communication. For the CISSP exam, you need to understand the order of the layers, what layer number is assigned to each layer name, and the function of each layer. Each layer performs a defined function for the layer above it and communicates with its peer layer over an interface.
The OSI model is an ISO standard. Contrary to the acronym, ISO stands for International Organization of Standardization, not International Standards Organizations.