A process is ready-to-run when the tool has all required parameter values. Processes that are ready-to-run are symbolized in color—input (or project) data elements are blue, tool elements are yellow, and output data (derived data) elements are green, as shown below.
If you run the model from within ModelBuilder, the tool and derived data elements are displayed with drop shadows, indicating that the process has run and the derived data has been generated, as shown below.
The not-ready-to-run state means that the model cannot be run within ModelBuilder. If your empty data or value variables are exposed as model parameters, you can run the model by opening its dialog from the ArcToolbox window, since the user of the dialog will provide valid values. The example below, from Creating a simple model, cannot be run from ModelBuilder but can be run from ArcToolbox since the Output Table variable is a model parameter, which is all that is needed to run the tool.