You instantiate a module to create an occurrence of that module. For example, instantiating a resistor model will give you instances of that module, say R1, R2, etc. Here is an example of how to instantiate a resistor module:

resistor #(50) R1 (n1, n2);

Thus, you give the module name, an optional parameter list, an instance name, and an optional pin list. In this case both the parameters and pins are given by order. It is also possible to give them by name:

resistor #(.r(50)) R1 (.p(n1), .n(n2));