The plant is the system (process or device) you intend to control. The following figure shows a schematic example.
A Model Predictive Control Toolbox™ design requires a plant model, which defines the mathematical relationship between the plant inputs and outputs. The controller uses it to predict plant behavior.
The toolbox software requires the model to be linear, time invariant (LTI). You can define such a model as follows:
Create a transfer function, state space, or zero/pole/gain model using methods provided by the Control System Toolbox™ software
Derive it from plant data using, e.g., methods provided by System Identification Toolbox™ software
Derive it by linearizing a Simulink® model