I have posterior placenta. what does it mean?

most people have posterior placenta. its completely normal.

in your case placenta is located on back wall of uterus. in many it's on the front wall or anterior.
whether it's anterior or posterior it doesn't affect growth or development of baby. placenta takes nourishment and oxygen to baby through the cord.

What this means is that the placenta is on the back wall of the uterus. It could be on the top which is fundal in the front which is anterior and on the cervix which is called previa. With a posterior or fundal placenta it is much easier to.count fetal kicks and movements.