Can anyone tell what this mean please? "breast tissue is heterogeneously dense, which may obscure detection of small masses"

Normal. It sounds like a mammogram report. Detection of masses on a mammogram is easy in women whose breast tissue has been replaced in various proportions by fatty tissue - which happens with age. Denser breasts means there is less fatty tissue and more fibro-glandular tissue, more common in younger women, or older women who take hormones. A mass is easier seen in breasts that are not dense.
All mammograms are. Really just overlapping shadows based on differences in tissue density - soft tissue and fibrous tissue are more dense than fatty tissue. Some women naturally have more dense tissue. The larger amount of dense tissue makes it harder to see, or obscures, possible abnormalities. All mammograms can be graded by the background density - fatty; scattered; heterogenously dense; and dense.
Test not worthwhile. Your mammogram test is not helpful in making a diagnosis on you because your breast tissue is very thick.