And this Astrophage it’s the bacteria that they need to defeat to save the sun?

Yes, Astrophage is the microorganism causing the sun to lose energy, threatening Earth’s survival. Ryland Grace’s mission is to understand Astrophage and find a way to stop or counteract its effects to save the sun and prevent Earth's extinction. So, in essence, Astrophage is the "enemy" microbe they need to defeat or neutralize to save the sun.