What use of 'sum' in this line code?

[Robert]

Hi,

I find below code snippet on line:


//////////
m = 10
theta_A = 0.8
theta_B = 0.3
theta_0 = [theta_A, theta_B]

coin_A = bernoulli(theta_A)
coin_B = bernoulli(theta_B)

xs = map(sum, [coin_A.rvs(m), coin_A.rvs(m), coin_B.rvs(m), coin_A.rvs(m), coin_B.rvs(m)])
/////////

I see 
[coin_A.rvs(m), coin_A.rvs(m), coin_B.rvs(m), coin_A.rvs(m), coin_B.rvs(m)]

is simply a list, but I don't know what use of 'sum' in this line.
I replace the random number with a simple list:
///////
yy=map(sum, [13, 22, 33, 41])

In [24]: yy
Out[24]: [13, 22, 33, 41]
///////

I don't see 'sum' has any effect above.
The code source is from:
#http://people.duke.edu/~ccc14/sta-663/EMAlgorithm.html


What could you help me on the 'sum'?

Thanks,