An AuxKernel computes values that are stored in "Auxiliary Variables". There is an infinite number of ways to generate these values but some common ones are: based on other variables/data in the simulation (i.e. coupled data), read from an external file and interpolated from a Function. The Auxiliary system is _extremely_ flexible on purpose. The data values computed by AuxKernels and stored in Auxiliary Variables (AuxVariables) are often used for visualization (i.e. written to output files) but can also be coupled back into other calculations (including in Kernels) or provided as the input to other systems such as Postprocessors.

For more information: see the documentation for the Auxiliary System