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
Most heritability estimates have been based on family (and twin) studies. An approach developed more recently ([PMID 25383972]) estimates heritability based on unrelated individuals of mixed ethnic ("admixed") backgrounds. This approach appears to indicate lower heritability than is estimated from twin studies; for example, heritability estimates for height and body mass index are 0.55 and 0.23, respectively, compared to the ~0.80 and ~0.50 estimates from twin studies. If heritability is lower than classically thought, the proportion of genetic variance explained by SNP studies is actually significantly higher - relatively common SNPs may explain most genetic variation.
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; an independent source of information about heritability in humans (as well as other species) can be found at H2DB.
[PMID 25985137] Meta-analysis of the heritability of human traits based on fifty years of twin studies.