そう, for our purposes today, means, "That's right" or "That's so" (the latter being an easy way to remember)
This useful word is used in various idiomatic ways. It is best to learn each as an example by heart.
Yes, that is right.
sou desu ka?
Is that so?; Really?; You don't say?
Now that you mention it; Speaking of that...
sou kangaeru to
Seen from that light; Thinking like that; From that point of view
If you do it that way...; if done