I am learning shell script. and i have this fibonacci sequence code. Can someone explain each line in the code to me and what it does please?

#!/bin/bash
	if [ $# -eq 1 ]
	then
	Num=$1
	else
	echo -n "Enter a Number :"
	read Num
	fi
	f1=0
	f2=1
	echo "The Fibonacci sequence for the number $Num is : "
	for (( i=0;i<=Num;i++ ))
	do
	echo -n "$f1 "
	fn=$((f1+f2))
	f1=$f2
	f2=$fn
	done
	echo