A: In its basic form, it means that you have assumed the risk of injury based upon the activity you have chosen to undertake.
Let's say you were skateboarding down the street and your skateboard hit a defect in the sidewalk causing you to fall and break your leg. When you bring a lawsuit against the town or municipality claiming they did not fix the dangerous condition in the sidewalk, their attorney will likely argue that you assumed the risk inherent with skateboarding, which they will argue, is a dangerous activity.
Once they make that claim, they must then prove it later at trial.
This is a powerful affirmative defense that basically says “Listen, you engaged in an inherently dangerous activity, so now you cannot be surprised when you suffered harm because of activity.” The same is true for any inherently dangerous sport or activity that winds up causing you injury.
However, keep in mind that even if you engage in dangerous activity that may cause risk of injury, the defense cannot absolve themselves of responsibility if in fact they were careless and that carelessness caused or contributed to your injury.
What this means is that if you also caused or contributed to your own injury, then the percentage of fault will be divided in some proportion between the actual wrongdoer and yourself.