Anything accidental happens by chance or unintentionally. People don't mean to do accidental things.
An accident is something unforeseen that no one wanted to happen, like a car accident or getting hit by lightning. You can describe anything like that as accidental. Bumping into a stranger could be accidental. Forgetting to pay a bill is accidental. This word covers all the things that no one really meant or planned to happen. There are a lot of accidental events in the world, which is one reason insurance exists.