python - what does n += 2 in this def?

I have a def function looks like:

def s(xs, n, m):
    t = []

    while n < m:
        t.append(xs[n])
        n += 2
    return t

I understand the above code by t.append(xs[n]), but have no idea what n += 2 means here.

Any help will be appreciated.
Thanks