This means the first mammogram showed an abnormal area in the breast, but further tests did not find a problem. The first mammogram result is called a false-positive result. You will not need to have any further tests or treatment. You will be invited back for a routine mammogram in three years (depending on your age).
Many false-positive results are caused by tiny deposits of calcium in the milk ducts. This is known as microcalcification. This happens in many women over the age of 50 and is usually harmless.
We have more information about breast calcification.