有人可以向我解释第4行是做什么的吗？
1 def fib2(n): # return Fibonacci series up to n
2 ... """Return a list containing the Fibonacci series up to n."""
3... result = []
4... a, b = 0, 1 #this line
5... while a < n:
6... result.append(a)
7... a, b = b, a+b
8... return result