What does "probably benign" mean on a mammogram report?

Most likely normal. While mammograms are excellent screening tests for cancer, they are not perfect. Many times spots are seen on mammography that are not normal but have such a low probability of being cancer (<0.5%), that it is best to watch these closely rather than biopsy. Radiologists categorize this as a birads-3 abnormality; when women get their first mammogram, there is a 7% chance this will happen.
6-month follow-up. Usually this is a lesion that has a very low risk of malignancy that the mammographer wants to follow closely (typically at 6-months, but sometimes sooner). According to the birads manual, the system radiologists use to manage mammography patients, the radiologist should think there is <2% risk of malignancy. In reality, the actual risk of malignancy is <1% (0.5-0.8 are numbers I have seen quoted).
Probably benign. In terms of BIRADS categories, it is category 3, technically less than 2% chance of cancer, but in practice it is less than 1%. Usually short interval follow up is recommended.