Explanation of a C code: pp, &sPID, memset

#include <string.h> 
#include <stdio.h> 
/*===================================================== 
PID Function 

The PID function is used in mainly 
control applications. PIDCalc performs one iteration of the PID 
algorithm. 

While the PID function works, main is just a dummy program showing 
a typical usage. 
======================================================*/ 

typedef struct PID { 

double SetPoint; // Desired Value 

double Proportion; // Proportional Const 
double Integral; // Integral Const 
double Derivative; // Derivative Const 

double LastError; // Error[-1] 
double PrevError; // Error[-2] 
double SumError; // Sums of Errors 

} PID; 

/*===================================================== 
PID 
======================================================*/ 

double PIDCalc( PID *pp, double NextPoint ) 
{ 
double dError, 
Error; 

Error = pp->SetPoint - NextPoint; 
pp->SumError += Error; 
dError = pp->LastError - pp->PrevError; 
pp->PrevError = pp->LastError; 
pp->LastError = Error; 
return (pp->Proportion * Error 
+ pp->Integral * pp->SumError 
+ pp->Derivative * dError 
); 
} 

/*===================================================== 
Initialize PID Structure 
======================================================*/ 

void PIDInit (PID *pp) 
{ 
memset ( pp,0,sizeof(PID)); 
} 

/*===================================================== 
Main Program 
======================================================*/ 

double sensor (void) // Dummy Sensor Function 
{ 
return 100.0; 
} 

void actuator(double rDelta) // Dummy Actuator Function 
{} 

void main(void) 
{ 
PID sPID; // PID Control Structure 
double rOut; // PID Response (Output) 
double rIn; // PID Feedback (Input) 

PIDInit ( &sPID ); // Initialize Structure 
sPID.Proportion = 0.5; // Set PID Coefficients 
sPID.Integral = 0.5; 
sPID.Derivative = 0.0; 
sPID.SetPoint = 100.0; // Set PID Setpoint 

for (;;) { // Mock Up of PID Processing 

rIn = sensor (); // Read Input 
rOut = PIDCalc ( &sPID,rIn ); // Perform PID Interation 
actuator ( rOut ); // Effect Needed Changes 
} 
}

double PIDCalc( PID *pp, double NextPoint ) 
{ 
double dError, 
Error; 

Error = pp->SetPoint - NextPoint; 
pp->SumError += Error; 
dError = pp->LastError - pp->PrevError; 
pp->PrevError = pp->LastError; 
pp->LastError = Error; 
return (pp->Proportion * Error 
+ pp->Integral * pp->SumError 
+ pp->Derivative * dError 
); 
}

void PIDInit (PID *pp) 
{ 
memset ( pp,0,sizeof(PID)); 
}

void main(void) 
{ 
PID sPID; // PID Control Structure 
double rOut; // PID Response (Output) 
double rIn; // PID Feedback (Input) 

PIDInit ( &sPID ); // Initialize Structure 
sPID.Proportion = 0.5; // Set PID Coefficients 
sPID.Integral = 0.5; 
sPID.Derivative = 0.0; 
sPID.SetPoint = 100.0; // Set PID Setpoint 

for (;;) { // Mock Up of PID Processing 

rIn = sensor (); // Read Input 
rOut = PIDCalc ( &sPID,rIn ); // Perform PID Interation 
actuator ( rOut ); // Effect Needed Changes 
} 
}