It probably means something like this:
You want to convert the seconds to hours, minutes and seconds. So 3601 secs would be 1h 0m 1s. So you have 3input and you want to give 1 output.
In C there are two ways to actually store the outputs with a function. Those are
considering that you want to save a value of type. You will save that value inside a variable names output.
output = value;
output = fun();
void fun(int* output)
*output = value;
You will use the second method. To hint it should be like
where in1, in2, in3 are you input and out your output, of course you would need to put types on them.
void fun(in1, in2, in3, out);
If you need any clarifications just keep posting 'till you figure it out.
EDIT: The & symbol means that you get the address. You could write the second like this to be more clear
outPtr = &output; //point to variable
fun(outPtr); //the variable output has the value you want, since it where outPtr points