Python: next() function

I'm learning Python from a book, and I came across this example:

M = [[1,2,3],
     [4,5,6],
     [7,8,9]]

G = (sum(row) for row in M) # create a generator of row sums
next(G) # Run the iteration protocol

Since I'm an absolute beginner, and the author hasn't provided any explanation of the example or the next() function, I don't understand what the code is doing.