Heritability is a word often used in popular science articles; however it is frequently misunderstood, even by scientists. Heritability is the difference, or variance, of a trait between people that can be attributed to differences in genetic factors. To phrase this in another way, heritability does not explain the fraction of trait caused by genes, but the impact of genes on the difference of a particular trait between you and the person next to you. 

This means that traits that are the same for all or most humans, such as having 5 fingers, have a heritability of almost 0! This is not to say that having 5 fingers does not have a genetic basis, merely that there are little to no population variation in this trait caused by genetic factors.
The causes of the differences of a certain trait between people can be broken down into three sources: genetics (or heritability), shared environment, e.g. familial or neighborhood effects, or unshared environments, e.g. life events or personal interests. These types of studies are often done using twins as they are either genetically identical or share many of the same environmental factors. So, if the trait is more similar in identical twins compared to fraternal (genetically different) twins then we know that there is likely more of a genetic basis, rather than environmental, to a trait.
When we say that a trait, such as Body Mass Index (BMI), has .39 heritability, we mean that 39% of the difference in BMI between you and another person is due to genetic factors, while the rest is due to environment, such as exercise or personal eating habits. Thus, rather than seeing heritability as the ultimate deterministic system, know that there is still space to influence your future and plenty of reasons to eat your vegetables.