I think (not certain but educated guess) it just means you have some dense tissue in your breasts. That's not uncommon but they note it (and in my state if you have dense breasts they send a letter making sure you know that I think....that was so many letters and scans ago now) because it is harder to feel lumps in dense breasts and is more important to keep up with mammograms. One of the reasons I have so many biopsies is that my breasts are so dense physical exams don't show much and so they need imagining and then the imaging isn't clear and so they biopsy and it usually is what they think it is. I don't think they are saying you have very dense breasts; I think they call that hyperdensity or something like that, just you have spots of density both in the tissue and the breast glands.