Poorly differentiated refers to how the cancer cells look under a microscope. It means that the cells don't look much like normal breast cells.
As normal cells develop, they differentiate. This means they change and become specialised, depending on where in the body they are. Breast cells look different from liver cells, for example.
Cancer cells are not fully differentiated. They can be almost like normal cells and are then said to be well differentiated. If they are very unlike normal cells they are called poorly differentiated.
Your doctor might talk to you about the grade of your cancer. In general, the more the cells look like normal cells, the lower the cancer grade. And the less normal they look, the higher the grade.
With some types of cancer doctors use grade as one of the factors to decide on the treatment you need.