A piece of bad luck or a less favorable position is a disadvantage. If you are trying to run a fifty-yard dash in flip flops when everyone else has on running shoes, you'll be at a disadvantage.
It's harder to be successful when you start with a disadvantage. One kind of disadvantage is being born into a poor family — it's a struggle for a child in poverty to do well in school, attend college, or end up with a well-paying job. A disadvantage is the opposite of an advantage, a lucky or favorable circumstance. At the root of both words is the Old French avant, "at the front."