The program contains a declaration of a class called HelloWorld with two fields and one equation. The first field is the variable x which is initialized to a start value 1 at the time when the simulation starts. The second field is the variable a, which is a constant that is initialized to 1 at the beginning of the simulation. Such a constant is prefixed by the keyword parameter in order to indicate that it is constant during simulation but is a model parameter that can be changed between simulations.
The Modelica program solves a trivial differential equation: x' = - a * x. The variable x is a state variable that can change value over time. The x ' is the time derivative of x.
Plot the results.