Code:
var compute_days(var age)
{
   return age * 365.25;
}

You are creating a function that accepts a var argument. This argument will be known as age inside the function(you declared it as so). The functin will return another var resulting from the calculation declared inside your function.

Code:
function main()
{
   var my_age = 33;
   var number_of_days = compute_days(my_age);
   printf("I am %.f days old!",number_of_days);
}

You declare a var named my_age.
You call the function declared above and pass in the my_age variable (Inside the function it will be known as age and will have the value you passed in when you called the function) and put the value inside the number_of_days variable.
Finally, you print it.

Best regards.

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