Understanding my Report/Mammogram

I was just told I had an abnormal Mammogram.  They said I have to go for an ultrasound.  I am very scared.  It said on the report it is my right breast.  It also said this:
"ACR-Category 0"  It also said I had denseness.  I do have large breasts.  What does that mean?  Does that mean it is serious?

[CCF-RN,MSN-JS]

Dear summerdaisy, Breasts are made up of ducts, lobes and fat.  This tissue is also called parenchyma.  When a mammogram is done it shows a two-dimensional view of a three-dimensional structure like an x ray.  Breast tissue is very dense and shows up white on a mammogram.  Fat, which is not very dense, shows up as gray.  A younger person