What does mucin secreting adenocarcinoma mean?

It is a cancer: Many cancers can form glands...So they are called adenocarcinoma. Further, some of these glands function like normal glands and can secrete mucin, so it is just a feature which has no particular implications beyond saying it is an adenocarcinoma.

Makes mucous : This basically describes a tumor (adenocarcinoma) that produces mucin (mucous) and excreted it outside the cells. Many will be found to have it only inside the cells or barely distinguishable at all. It is hard to comment on prognosis without knowing many other facts.