Post-partum depression

FTM Here and wondering what causes/is ppd? I mean I've seen people on tv/movies with it and hear about people having it I'm just wondering how common is it? What causes it? How does it make you feel etc? I'm not wondering for myself as I am early in my first pregnancy but I start to think about how far I've come in just a couple of months and how this journey of pregnancy is an emotional ride, when it's over I can totally understand feeling a sense of, i dont know the right word? "Loss" I guess because its not the same as having your baby growing inside you. I just want to get educated early on about as much as pregnancy and motherhood as possible as that is going to be the most important role of my life!