Girls, explain to me what the screening (blood test) is all about - I …

Girls, explain to me what the screening (blood test) is all about - I asked and they said that deviations will show if there are any, but what does it mean? And also, if the first one is good, then do you need the second one? I didn't have such a thing during my first pregnancy, it was 11 years ago. I feel like I'm lagging behind, please explain. Thank you. They said to start with an ultrasound, then show the ultrasound at the clinic, and write a referral for a screening in a private lab I booked an ultrasound.