An infinite series is the sum of an infinite number of terms, $$a_1 + a_2 + a_3 + \ldots,$$which we also write as $$\sum_{n=1}^\infty a_n.$$ But addition is something we do to finitely many numbers, not to infinitely many!
Once again, we can use limits to make sense of such a sum, just as in the definition of improper integrals of type I:
we can add up finitely many terms and then take a limit.
Definition:
$$\sum_{n=1}^\infty a_n = \lim_{n \to \infty} s_n,$$where$$s_n = \sum_{i=1}^n a_i$$is called the n-th partial sum.
Note: There is an error at the 8:05 mark of the video. The voice-over is
correct, but what is written is a little different. The correct values are
$s_4=-2$, $s_5=3$ and $s_6=-3$. This doesn't affect the
point being made about divergence.