Heritability is defined as the degree to which individual genetic variation accounts for phenotypic variation seen in a population. In a scientifically sloppy sense, it's somewhat (mis)used to give a sense of what proportion of your risk for a given disease is derived from your genes (as opposed to non-genetic influences, such as your environment, diet, etc). Wikipedia
It is important to remember that high heritability does not imply that a trait or disease is inevitable. In fact, some highly heritable traits can be modified by simple changes in the environment. Phenylketonuria (PKU), for example, is a genetic disease with high heritability that can be strongly influenced by a relatively straightforward change in diet (restricting intake of the amino acid phenylalanine). Even if Alzheimer's disease has a relatively high heritability, environmental factors and changes may alter disease progression and severity.
Heritability estimates for selected medical conditions present in SNPedia are summarized below: