It's hard to describe, but it's literally thicker than the surrounding skin. It's pretty easy to tell it's thicker, by simply feeling it. Sometimes it's also dimpled like the peel of an orange. Depending on the cause, it may also be red, and there may or may not be a lumpy feeling underneath it. Many women have thickend skin at the inferior fold of the breast, because of the rubbing of the overhanging breast tissue, and often there's a thick ridge of tissue under the skin in that location. As with any other breast issue, if a woman thinks she's noticed something different or unusual about her breast, she ought to have it checked.
Dear shelbyjosh, "Skin thickening" could be referring to a less distinct area of firmness in an area of the breast, rather than a distinct lump. It might be describing a raised area of skin. Sometimes on a mammogram report there might be mention of thickening. Taken out of context it is difficult to say anything in regard to a relationship to breast cancer.