New to bbt...can someone explain?

So we've been ttc baby 3 since last may. I've always used opk and that's it to track ovulation.

Since I'm nursing I'm ovulating around CD 18-20 and my luteal phase is about 7 days so my dr gave me clomid to start days 5-9 my next cycle along with a bbt chart.

I'm logging my bbt but what does any of this mean? How will I know the day of ovulation? All of this looks so confusing and intimidating.