Glucose test

Brittany • 19. Engaged to the love of my life. Been together for 2.5 years. 2 amazing step kids. And a ftm , to our son who was born February 3, 2016. ❤
Okaa so I am a ftm , I'm 28weeks1day and people keep asking if I've had a glucose test done. I say no , bc I really haven't had one done. But I was wanting to know , what's the point in this test? What is it for? Why do they test for it? And how to read the results?