Posterior placenta is the position of placenta in relation to the uterus. Placenta can be anterior or posterior. It can be in upper segment, midsegment or lower segment . Once you reach your third trimester the placental position will be mentioned in terms of segments. So as of now you don't need to worry about the placenta when it is mentioned posterior.
Next Steps
In Follow up scans look for position of placenta and which segment it belongs
Health Tips
If it is a low lying placenta then you need to
1) Avoid lifting weights
2) Avoid sexual intercourse
3) Avoid strenuous exercise
4) Avoid long distance travel