real analysis - Two infinite series converge, prove product converges

If

$$\sum_{k=1}^\infty a_k^2$$
and
$$\sum_{k=1}^\infty b_k^2$$
are both convergent.
Prove that the following infinite series is convergent:
$$\sum_{k=1}^\infty a_kb_k$$

$\\$

I am not fully sure how to go about this. This is all the info given in the problem and I think I'm just missing some sort of fundamental concept. Could anyone please explain the general idea behind this? I'd appreciate if you didn't just explicitly give me the answer, but maybe just a foundational piece to get me going as I really want to get this one by myself!