What does endometrial cavity echo measures 9 mm in bilayer thickness mean?

It's a good thing! They are measuring the lining inside the uterus (the lining is the stuff that comes out with your period). It is 9 mm's wide, which is normal for someone your age. There are 2 layers to the lining, one from the top wall and one from the bottom wall, so that is why they call it "bilayer".
Normal. For a 31 year old, that's pretty normal.