Communication and Synchronization
When you call glDispatchCompute() (or glDispatchComputeIndirect()), a potentially huge amount of work is sent to the graphics processor. The graphics processor will run that work in parallel if it can, and the invocations that execute the compute shader can be considered to be a team trying to accomplish a task. Teamwork is facilitated greatly by communication and so, while the order of execution and level of parallelism is not defined by OpenGL, some level of cooperation between the invocations is enabled by allowing them to communicate via shared variables. Furthermore, it is possible to sync up all the invocations in the local workgroup so that they reach the same part of your shader at the same time.
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