Rackets and strings, how do they work? How do low and high tension affect the game? What are the general principles?

I've been playing for a year and a half. I'm low-intermediate. So far rackets and strings are like black magic to me, I don't know anything and I'd like to change that and get a general understanding of how they work and how to choose them. If anyone can recommend maybe an online article or doesn't mind explaining a bit, it'd be much appreciated. Thanks!